Municipal Bond Interest: Federal Tax Exemption Under IRC 103

Interest paid on most bonds issued by states, cities, counties, school districts, and other local governments is excluded from federal income tax under Internal Revenue Code Section 103(a).¹ That is the core of the municipal bond tax exemption, and it has been part of federal law since the Revenue Act of 1913. The rule is narrower than it sounds, though. It shelters the periodic interest payments only, and even that shelter can be pierced by the alternative minimum tax, undone by how a bond is purchased or sold, and offset by ripple effects on Medicare premiums and Social Security taxation.

What the Exemption Actually Covers

Section 103(a) excludes interest, and nothing else, from your gross income. Every other dollar a municipal bond generates is treated on its own terms.

Capital Gains Are Still Taxed

Sell a muni for more than you paid and the profit is a capital gain. Long-term rates run from 0% to 20% depending on your income and filing status.² The tax-free status of the coupon has no bearing on that gain.

Original Issue Discount

When a bond is originally sold below face value, the gap is called original issue discount. On a tax-exempt bond, accrued OID is generally not federally taxable, but you still have to track it. When you sell or redeem the bond, the accrued OID gets added to your cost basis as though it had been taxable. Skip that adjustment and you can overstate a loss or understate a gain.³

Market Discount

Market discount works differently. It arises when you buy an existing bond on the secondary market for less than its adjusted issue price, usually because rates have risen. If the discount exceeds a de minimis threshold of 0.25% of face value multiplied by the number of full years remaining to maturity, the accrued discount is taxed as ordinary income on sale or redemption. Anything at or below the threshold gets capital gains treatment. A bond with 10 years to maturity, for instance, has a threshold of 2.5% of face, so a $97.50 purchase on a $100 par bond stays in the safe harbor.

Which Bonds Qualify

The exemption reaches obligations issued by a state, a U.S. territory, or a political subdivision such as a county, city, school district, or municipal utility district.¹ A political subdivision must exercise at least one substantial governmental power: taxation, eminent domain, or police powers. Private loans and corporate debt never qualify, even if a local government is loosely tied to the project.

Two broad categories dominate the market. General obligation bonds are backed by the issuer’s full faith and credit, meaning its taxing power stands behind the payments. Revenue bonds are repaid only from a specific income stream: tolls, utility fees, or lease payments from the financed asset. Either type can qualify under Section 103.

The Private Activity Bond Trap

Not every bond issued by a local government pays tax-free interest. Section 103(b)(1) strips the exemption from any private activity bond that is not also a “qualified bond” under Section 141.¹ A bond becomes a private activity bond when it flunks two tests at the same time:

More than 10% of the proceeds are used for a private business purpose.⁴
More than 10% of the debt service is secured by, or paid from, property used in that private business.⁴

Both thresholds drop to 5% for private use that is unrelated to the governmental purpose the bond was issued to finance.⁴

Congress carved out specific categories that keep their exemption even as private activity bonds: airports, docks and wharves, water and sewage facilities, solid waste disposal, qualified residential rental projects, qualified mortgage programs, and bonds benefiting Section 501(c)(3) nonprofits. Issuers must monitor the private-use percentage over the life of the bonds. If the limits are breached later, the IRS can retroactively declare the interest taxable, hitting every investor who bought expecting tax-free income.

AMT on Specified Private Activity Bonds

Here is the surprise that catches many investors. Even when a private activity bond qualifies under Section 103, its interest can still trigger the alternative minimum tax. Under Section 57(a)(5), interest on “specified private activity bonds” issued after August 7, 1986, is treated as a tax preference item and added back when calculating alternative minimum taxable income.⁵ The coupon is excluded from your regular income tax and can still raise your AMT bill.

Several categories escape the AMT add-back:

501(c)(3) bonds issued for qualifying nonprofits.
Qualified mortgage bonds, qualified veterans’ mortgage bonds, and exempt facility bonds where at least 95% of proceeds fund qualified residential rental projects, provided the bonds were issued after July 30, 2008.
Refunding bonds where the original bond was issued before August 8, 1986.
New-money bonds issued during calendar years 2009 and 2010.

This becomes more consequential starting in 2026. Provisions of the Tax Cuts and Jobs Act that raised AMT exemption amounts and phaseout thresholds are scheduled to expire after 2025. If they do, the AMT will reach a larger pool of taxpayers, and anyone holding specified private activity bonds should reassess exposure. Your broker reports this interest separately on Form 1099-INT, Box 9.⁶

Reporting and Other Federal Tax Effects

Tax-exempt does not mean invisible. You report all municipal bond interest on Line 2a of Form 1040, even though it stays out of taxable income on Line 2b.⁷ Your Form 1099-INT reports tax-exempt interest in Box 8 and specified private activity bond interest in Box 9.⁶

Section 265 blocks the deduction of interest on money you borrowed to buy or carry tax-exempt bonds.⁸ Take out a margin loan and buy munis with it, and the margin interest is not deductible.

Medicare Premium Surcharges

Medicare Part B and Part D premiums are income-adjusted through the Income-Related Monthly Adjustment Amount, or IRMAA. The formula uses modified adjusted gross income, which includes tax-exempt interest. For 2026, a single filer with MAGI above $109,000, or a joint filer above $218,000, starts paying surcharges that can reach $487 per month for Part B and $91 per month for Part D at the highest tiers.⁹ A large muni portfolio can push a retiree over these thresholds even though none of the interest is federally taxable.

Social Security Benefit Taxation

Whether Social Security benefits are taxed depends on “provisional income,” which is adjusted gross income plus half of your benefits plus all tax-exempt interest. If provisional income exceeds $25,000 (single) or $32,000 (joint), up to 50% of benefits become taxable. Above $34,000 (single) or $44,000 (joint), up to 85% can be taxed.¹⁰ Those base amounts have not been adjusted for inflation since 1984, so they catch more retirees each year. Municipal interest, though “tax-free,” can be the factor that pushes benefits into the taxable range.

State Income Tax Is a Separate Question

Federal exemption does not carry over automatically to your state return. Most states exempt interest on bonds issued within their own borders and tax interest on out-of-state munis like any other income. A few states have no income tax, making the question moot. Others exempt all municipal interest regardless of origin. Some, such as Illinois, tax even their own in-state municipal interest in certain situations. Check your state’s specific rules before assuming your muni portfolio is fully tax-free at the state level.

1
Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds
2
Internal Revenue Service. Topic No. 409, Capital Gains and Losses
3
Internal Revenue Service. Publication 1212, Guide to Original Issue Discount (OID) Instruments
4
Office of the Law Revision Counsel. 26 USC 141 – Private Activity Bond; Qualified Bond
5
Office of the Law Revision Counsel. 26 USC 57 – Items of Tax Preference
6
Internal Revenue Service. Instructions for Forms 1099-INT and 1099-OID
7
Internal Revenue Service. U.S. Individual Income Tax Return (Form 1040)
8
Office of the Law Revision Counsel. 26 USC 265 – Expenses and Interest Relating to Tax-Exempt Income
9
Centers for Medicare & Medicaid Services (CMS). 2026 Medicare Parts A and B Premiums and Deductibles
10
Internal Revenue Service. Publication 915, Social Security and Equivalent Railroad Retirement Benefits