Your broker shows you two bonds. A corporate bond pays 5.1%. A municipal bond from your own state pays 3.4%. The muni looks like a worse deal by nearly two full percentage points — until you remember the IRS never touches the muni's interest. If you sit in the 32% federal bracket, that 3.4% keeps every penny while the 5.1% keeps only about 3.5% after tax, and the muni wins outright once your state tax joins the math. But the tax-free promise has fine print: buy the wrong kind of muni, or the right kind at too steep a discount, and part of your return gets taxed as ordinary income. This guide covers how the exemption works, the two bond structures you are choosing between, the one formula that makes comparisons honest, and the two traps that surprise buyers most.
Why Muni Interest Escapes Federal Tax
Interest on most state and local government bonds is excluded from federal gross income under Section 103 of the Internal Revenue Code. The logic dates to the founding compromise between federal and state taxing powers: the federal government does not tax the borrowing cost of states and cities, which lets those issuers borrow more cheaply. You still report the interest — it appears on your return as tax-exempt interest — but it does not enter the income your federal rate applies to.
State treatment is a second, separate question. Most states exempt interest on bonds issued by their own state and its localities, so an in-state bond can be free of both federal and state tax. A few states break the pattern and tax even their own bonds' interest, so confirm your state's rule rather than assuming. Interest on another state's bonds is generally taxable in your home state. Bonds issued by U.S. territories — Puerto Rico, Guam, the U.S. Virgin Islands — are exempt from federal, state, and local tax everywhere, which is why so-called triple-exempt funds hold them.
Know what the exemption does not cover. It shelters interest, not gains. If you sell a muni above your cost basis, or hold a bond bought below par to maturity, the profit is taxable — and, as the de minimis section below shows, sometimes at ordinary rates rather than capital-gain rates. Original-issue-discount bonds have their own accrual rules. The exemption is generous, but it is an interest exemption, not an invisibility cloak for the whole investment.
The Two Kinds: General Obligation vs. Revenue Bonds
Every muni falls into one of two security structures, and the difference decides what stands behind your interest payments.
General obligation (GO) bonds: backed by taxing power
A GO bond is secured by the issuer's full faith, credit, and taxing power. The city, county, school district, or state pledges to levy whatever taxes are needed to pay you. Because the repayment source is the entire tax base rather than one project, GO bonds are generally considered the safer of the two structures, and they usually pay lower yields to reflect that.
Safety is relative, not absolute. A shrinking tax base, pension burdens, or a legal cap on property-tax levies can strain even a GO pledge — some GO bonds are "limited-tax," meaning the issuer can only raise taxes to a voter-approved ceiling, while "unlimited-tax" GO bonds carry no such cap. Before buying, check which kind you hold and glance at the issuer's credit rating and recent financial disclosures on the MSRB's EMMA system, the free public repository for muni filings.
Revenue bonds: backed by one project's cash flow
A revenue bond is repaid from the income of a specific project or system — toll collections on a highway, landing fees at an airport, water bills paid to a utility, patient revenue at a hospital. If the project underperforms, there is no tax base to fall back on. That concentrated risk is why revenue bonds typically yield more than comparable GO bonds.
Evaluate a revenue bond the way you would evaluate the business underneath it. A water utility with a monopoly service area and essential demand is a different credit from a speculative development district or a hospital in a competitive market. The official statement for the issue describes the pledged revenue, the rate covenant (the issuer's promise to keep fees high enough to cover debt service, often at a stated multiple), and any reserve fund. Read those three items before you read anything else.
Hybrids and what to choose
Some bonds blend the two: "double-barreled" issues are secured by project revenue with a GO pledge as backup. For most individual buyers the practical tradeoff is simple. If your priority is maximum safety and you accept a lower yield, favor high-rated GO bonds. If you are comfortable underwriting a specific enterprise for extra yield, revenue bonds are the tool — but diversify across issuers and sectors rather than betting on one toll road.
Tax-Equivalent Yield: The One Formula That Makes Comparisons Honest
A taxable yield and a tax-exempt yield cannot be compared face to face. Tax-equivalent yield (TEY) converts the muni's yield into the yield a taxable bond would need to pay to leave you with the same after-tax income:
TEY = Tax-exempt yield / (1 − your marginal tax rate)
Use your marginal rate — the rate on your next dollar of income — not your effective rate, because the bond's interest would stack on top of your existing income. If the bond is also exempt from state tax, fold that in with a combined rate: combined rate = federal rate + state rate × (1 − federal rate), which accounts for the fact that state taxes are no longer deductible for most filers under the SALT cap.
Worked examples
Take a muni yielding 3.4%:
- In the 24% federal bracket: 0.034 / (1 − 0.24) = 0.0447, or 4.47%. A corporate bond must beat 4.47% to win.
- In the 32% bracket: 0.034 / 0.68 = 5.00%. The 5.1% corporate bond barely edges it.
- In the 37% bracket: 0.034 / 0.63 = 5.40%. The muni wins by a clear margin.
Now add a 9% state tax with an in-state, doubly exempt bond for the 32% filer. Combined rate = 0.32 + 0.09 × 0.68 = 0.381. TEY = 0.034 / (1 − 0.381) = 5.49%. The state exemption adds nearly half a point of edge — which is why comparing an out-of-state muni fund to an in-state one on headline yield alone misleads.
The formula also runs in reverse. To find what a taxable bond keeps, multiply by (1 − rate): a 5.1% corporate bond in the 32% bracket keeps 5.1% × 0.68 = 3.47% after federal tax, versus the muni's full 3.4% — nearly a tie before state tax, a muni win after it.
The most common mistake is skipping this conversion entirely: investors see a lower muni coupon, assume taxable bonds pay more, and hold taxable bonds in taxable accounts for years while overpaying tax on every coupon. Run the division once per purchase. It takes thirty seconds and it is the whole ballgame.
Trap 1: Private-Activity Bonds and the Alternative Minimum Tax
Most muni interest is exempt under both the regular income tax and the Alternative Minimum Tax. The exception is interest from private-activity bonds — munis that finance projects primarily benefiting private parties, such as airport terminals leased to airlines, multifamily housing developments, student-loan programs, and nonprofit hospital expansions. The tax code treats that interest as a preference item for AMT purposes: if you are subject to the AMT, the interest gets added back into your AMT income calculation and can be taxed at the AMT rates of 26% or 28%.
Before 2018 this trap caught far more filers. The Tax Cuts and Jobs Act raised AMT exemptions and phaseout thresholds substantially, so far fewer individuals pay AMT today — but high earners with large state-tax deductions, incentive stock option exercises, or other preference items still land in it, and for them a portfolio full of private-activity bonds quietly surrenders part of its tax edge.
Protect yourself three ways. First, check the bond's official statement or your fund's disclosures: funds report the percentage of income from private-activity bonds, and many offer explicitly AMT-free portfolios. Second, if you have ever paid AMT or exercise incentive stock options, tell your advisor to screen for AMT exposure before adding munis. Third, remember that private-activity bonds usually yield slightly more than comparable governmental munis — that spread is the market pricing the AMT risk, so treat it as compensation for a risk you should verify you do not bear.
Trap 2: The De Minimis Rule on Bonds Bought Below Par
The second trap hides in bargain hunting. When you buy a muni on the secondary market below its face value, the discount — the market discount — is profit you realize at sale or maturity. Small discounts get capital-gain treatment. Large ones do not.
The dividing line is the de minimis threshold: 0.25% of face value multiplied by the number of full years remaining to maturity. If your discount is smaller than that amount, the gain is a capital gain. If it is equal to or larger, the accrued market discount is taxed as ordinary income — at rates up to 37% federally instead of the 0%, 15%, or 20% long-term capital-gain rates.
A worked example
You buy a $1,000 face-value bond with 10 full years left to maturity for $960 — a $40 discount, or 4%. The threshold is 0.25% × 10 = 2.5%, or $25. Your $40 discount exceeds it, so the entire $40 of market discount is taxed as ordinary income when the bond matures (it accrues ratably over the bond's remaining life unless you elect the constant-yield method). Had you paid $976 — a 2.4% discount, under the threshold — the $24 gain would have qualified for capital-gain treatment.
The practical consequences cut two ways. As a buyer, a bond quoted at a deep discount is not as cheap as it looks once ordinary-income tax on the accretion is priced in — compute your after-tax yield to maturity, not just the headline yield. As a seller sitting on a premium bond, the mirror image applies: amortized premium reduces your basis rather than generating a deduction on a tax-exempt bond, another detail worth confirming with your tax preparer before harvesting what looks like a loss.
Note the distinction between market discount and original issue discount. OID — the discount built in when the bond is first issued below par — generally accrues as tax-exempt interest for a tax-exempt bond. Market discount arises later, from price moves in the secondary market, and follows the harsher rule above. Two bonds with identical coupons and maturities can thus carry very different tax outcomes depending on when and where in the price history you bought.
Bookkeeping: Keep Tax-Exempt Interest Out of Your Taxable Buckets
Muni interest being tax-free does not mean it is record-free. You still need clean records: the IRS receives information returns on your tax-exempt interest, your state may tax out-of-state muni interest, and accrued market discount must be tracked year by year. The bookkeeping habit that prevents April surprises is separating tax-exempt interest from taxable interest at the moment you record it, not at tax time.
In a plain-text ledger, that separation is one account branch:
2026-07-01 * "Springfield GO bond - semiannual coupon"
Assets:Brokerage:Cash 850.00 USD
Income:Interest:TaxExempt:Muni -850.00 USD
2026-07-01 * "Corporate bond fund - monthly distribution"
Assets:Brokerage:Cash 412.50 USD
Income:Interest:Taxable:CorpBond -412.50 USDWith the two streams in distinct accounts, your year-end totals answer the return's questions directly: the TaxExempt balance is the tax-exempt interest line, the Taxable balance is ordinary interest, and nothing needs untangling from a commingled lump. If you hold an out-of-state bond whose interest your state taxes, add a third branch or a tag so the state adjustment writes itself. Track purchase price versus par in the lot's metadata while you are at it — that is the raw material for the de minimis computation. If you are new to plain-text accounting, the docs walk through opening accounts and recording investment income step by step.
A Buyer's Checklist Before You Place the Order
Run through these seven items for every muni purchase and both traps stay shut:
- Compute tax-equivalent yield at your marginal rate, including state tax when the bond is in-state exempt. Compare TEY to taxable alternatives, never coupon to coupon.
- Confirm the security structure. GO or revenue? Limited-tax or unlimited-tax? Which revenue stream, what rate covenant, what reserve fund?
- Check AMT status. Is it a private-activity bond? If you pay AMT or might, prefer AMT-free issues or funds and verify the fund's reported AMT percentage.
- Price versus par. If buying below face value, compute the de minimis threshold (0.25% × full years left) and price any ordinary-income accretion into your after-tax yield.
- Read the EMMA disclosures. Pull the official statement and continuing disclosures from the MSRB's EMMA system; confirm the credit rating and skim the risk factors.
- Match maturity to your horizon. Longer bonds pay more and swing more when rates move. A bond you must sell early can lose more in price than years of coupons pay.
- Fund or individual? Individual bonds held to maturity return principal on a known date; funds offer instant diversification and professional credit work but no maturity date. Small positions usually belong in a low-cost fund.
Start with item one. Pull up the muni you are considering, divide its yield by one minus your marginal rate, and see whether the "low" tax-free yield beats everything taxable on your shortlist. Most buyers who run that division once never compare bonds any other way again.
