Interest on a municipal bond is usually tax-free at the federal level under Section 103 of the Internal Revenue Code, but municipal bond interest becomes taxable in several specific situations: when the bond is a private activity bond that doesn’t qualify for a statutory exemption, when it’s an arbitrage bond, when it carries a federal guarantee, or when it isn’t issued in registered form.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds Even bonds that keep their federal exemption can generate an alternative minimum tax bill, be taxed by your state if issued elsewhere, or produce a taxable capital gain when you sell. Understanding which of these applies to a particular bond is what separates a tax-free coupon from an unwelcome line on your return.
The Default Rule Under Section 103
Section 103(a) says gross income does not include interest on any state or local bond. A “state or local bond” covers obligations of a state, the District of Columbia, any U.S. territory, or a political subdivision, meaning counties, cities, school districts, port authorities, utility commissions, and similar bodies that exercise governmental powers. Both general obligation bonds, backed by the issuer’s taxing power, and revenue bonds, backed by project income like tolls or water charges, fall under the exclusion.
Section 103(b) then carves out three categories that lose the exemption: private activity bonds that are not qualified bonds, arbitrage bonds, and bonds that fail the registration or federal guarantee rules.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds Each of these makes the interest fully taxable at ordinary federal rates.
Nonqualified Private Activity Bonds
A bond that a state or local government issues can still be reclassified as a private activity bond if too much of the benefit flows to a private business rather than the general public. Under Section 141, that happens along either of two paths.
The first path combines the private business use test and the private security or payment test. Both must be met: more than 10 percent of proceeds used in a nongovernmental trade or business, and more than 10 percent of debt service secured by or paid from property or revenue tied to that private use. Where the private use is unrelated or disproportionate to the government’s own use, the threshold drops to 5 percent.2Office of the Law Revision Counsel. 26 USC 141 – Private Activity Bond; Qualified Bond
The second path is the private loan financing test. If more than the lesser of 5 percent or $5 million of proceeds funds loans to nongovernmental borrowers, the issue is a private activity bond regardless of the other tests.
A state bond that builds a public highway stays tax-exempt because the project is governmental. A city bond that finances a stadium leased to a professional sports franchise typically flunks both tests, becomes a private activity bond, and, unless it fits a qualified category, produces taxable interest.
Private Activity Bonds That Keep the Exemption
Not every private activity bond is taxable. Section 103(b) only strips the exemption from private activity bonds that are not “qualified bonds,” and a large share of the municipal market consists of technically-private-activity bonds that satisfy a statutory carve-out and remain tax-free. Two categories dominate.
Exempt Facility Bonds
If 95 percent or more of the net proceeds fund one of the facility types listed in Section 142, the bond stays exempt even with a private operator involved. The list includes airports, docks and wharves, mass transit, water and sewer, solid waste disposal, affordable rental housing, local electric or gas facilities, broadband projects, and qualified carbon dioxide capture facilities, among others.3Office of the Law Revision Counsel. 26 USC 142 – Exempt Facility Bond A regional airport bond, even one where a private company runs day-to-day operations, is almost always still tax-exempt.
Qualified 501(c)(3) Bonds
Bonds issued for nonprofit hospitals, universities, and other 501(c)(3) organizations qualify when all bond-financed property is owned by the nonprofit or a governmental unit, and unrelated private business use stays under 5 percent of net proceeds.4Office of the Law Revision Counsel. 26 USC 145 – Qualified 501(c)(3) Bond Hospital revenue bonds are a huge segment of the muni market and stay tax-free under these rules.
Because “private activity bond” alone doesn’t decide the tax question, check the official statement or prospectus for the specific classification before assuming interest is taxable.
Arbitrage Bonds, Federal Guarantees, and Registration
Three other situations pull the exemption regardless of the private-use analysis.
- Arbitrage bonds. Section 148 defines these as bonds whose proceeds are reasonably expected to be used, directly or indirectly, to acquire investments yielding materially more than the bond itself pays. The rule prevents issuers from using tax-exempt borrowing as a spread-earning tool. Fail it, and the interest is taxable.5Office of the Law Revision Counsel. 26 USC 148 – Arbitrage
- Federally guaranteed bonds. A state or local bond loses its exemption if the federal government or a federal agency guarantees principal or interest. Section 149(b) preserves the exemption for specific programs, including FHA-insured mortgages, VA guarantees, and student loan guarantees, but federal backing outside those exceptions is disqualifying.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds
- Bonds not in registered form. A registration-required bond has to be issued in registered form, with ownership tracked through a central registry or book-entry system. Bearer bonds, transferred by physical delivery, do not qualify.
Practically, essentially all modern muni issues are book-entry securities held through the Depository Trust Company, so the registration rule rarely surfaces. Arbitrage compliance and federal guarantees are the exceptions that actually catch bonds.
AMT on Specified Private Activity Bonds
Even a private activity bond that keeps its regular federal exemption can produce taxable interest under the Alternative Minimum Tax. Interest on “specified private activity bonds,” defined as private activity bonds issued after August 7, 1986, whose interest is otherwise excluded under Section 103, must be added back as a preference item when computing alternative minimum taxable income.6Cornell Law Institute. Definition – Specified Private Activity Bond from 26 USC 57(a)(5)
For 2026, the AMT exemption is $90,100 for single filers and $140,200 for joint filers, with phase-outs starting at $500,000 and $1,000,000.7Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 Well below those thresholds, specified private activity bond interest usually won’t create AMT liability. In the phase-out band or above, it can. Bond offering documents disclose whether interest is an AMT preference item, and Box 9 of Form 1099-INT reports the amount separately at year end.8Internal Revenue Service. Instructions for Forms 1099-INT and 1099-OID
State Tax on Out-of-State Bonds
“Tax-free” on a muni bond generally refers to the federal level only. Most states exempt interest on bonds issued within their own borders and tax interest from bonds issued by other states. Buy a bond from another state and that interest normally appears as taxable income on your state return. State rates on this interest run from zero in states without an income tax up to roughly 13 percent in the highest-tax states, which is why investors in high-tax jurisdictions often stay in-state.
Territory Bonds
Bonds issued by the government of Puerto Rico and its authority are exempt from taxation by the federal government, all 50 states, and any local government in the United States.9Office of the Law Revision Counsel. 48 USC 745 – Tax Exempt Bonds That triple-exempt treatment applies wherever the bondholder lives. Bonds from Guam, the U.S. Virgin Islands, and American Samoa carry similar treatment under separate provisions. Credit quality among these issuers varies, so the tax benefit has to be weighed against the risk.
Capital Gains and Market Discount on Sale
Section 103 excludes interest, not gains. Sell a muni bond for more than you paid and the profit is a taxable capital gain: long-term rates if you held it more than a year, ordinary rates if not.
Market Discount
Buy a muni on the secondary market below face value and you’re holding market discount. When you sell or the bond matures, gain up to the amount of accrued market discount is taxed as ordinary income, not capital gain.10Office of the Law Revision Counsel. 26 USC 1276 – Disposition Gain Representing Accrued Market Discount Treated as Ordinary Income Only gain beyond the accrued market discount gets capital gains treatment. Buying at 95 and redeeming at par does not produce a tax-free return; the five-point pickup is taxable, with the market discount portion taxed at your ordinary rate.
The De Minimis Threshold
A small discount is treated as zero. Under Section 1278, if the market discount is less than one-quarter of one percent of face value multiplied by the complete years remaining to maturity, it’s ignored.11Office of the Law Revision Counsel. 26 USC 1278 – Definitions and Special Rules On a $1,000 bond with 10 years left, the cutoff is $25 (0.25% × $1,000 × 10). Purchase at $976 and you have taxable market discount; purchase at $978 and the discount falls under the line, so the eventual gain is a capital gain instead.
Bond Premium: No Deduction, But an Adjustment
Buy a muni above face value and you’re holding bond premium. Because the interest itself is not taxed, no deduction is allowed for the amortized premium on a tax-exempt bond.12Office of the Law Revision Counsel. 26 USC 171 – Amortizable Bond Premium You still reduce your cost basis each year by the amortized amount, and any period where amortization exceeds the interest produces a nondeductible loss.
When you report tax-exempt interest on your return, you reduce it by the year’s amortized premium. A bond paying $500 in annual interest with $50 of amortization is reported at $450 on Line 2a of Form 1040.13Internal Revenue Service. Instructions for Form 1040 – Tax-Exempt Interest Skipping the adjustment overstates tax-exempt interest and can distort the calculations that use it.
Reporting and the Social Security Effect
Tax-exempt interest still gets reported to the IRS. Your broker or paying agent shows the total in Box 8 of Form 1099-INT, and any specified private activity bond interest that’s an AMT preference item appears in Box 9 (already included in Box 8).8Internal Revenue Service. Instructions for Forms 1099-INT and 1099-OID The Box 8 figure, reduced by amortized premium, goes on Line 2a of Form 1040 and stays out of taxable income on Line 2b.13Internal Revenue Service. Instructions for Form 1040 – Tax-Exempt Interest
The Line 2a amount matters most for Social Security. The IRS calculates “combined income” as adjusted gross income plus tax-exempt interest plus half of Social Security benefits. If that number exceeds $25,000 for a single filer or $32,000 for a married couple filing jointly, part of the benefits becomes taxable.14Social Security Administration. Must I Pay Taxes on Social Security Benefits? Retirees with large municipal portfolios sometimes find that “tax-free” interest is quietly pushing Social Security benefits into the taxable range. The interest itself is still exempt, but the reporting flows into a calculation that isn’t.