Municipal Bonds: Tax-Free Income and What You Should Know

What Are Municipal Bonds?

Municipal bonds ("munis") are debt securities issued by states, cities, counties, and other government entities to fund public projects — schools, highways, water systems, hospitals. The defining feature: interest income is exempt from federal income tax and, in many cases, from state and local taxes as well if you live in the issuing state.

This tax treatment makes munis uniquely valuable for investors in high tax brackets holding bonds in taxable accounts.

Types of Municipal Bonds

General obligation (GO) bonds are backed by the full faith, credit, and taxing power of the issuer. Because the municipality can raise taxes to pay bondholders, GO bonds are generally considered the safest muni category — though safety varies enormously by issuer. Detroit filed for bankruptcy in 2013, the largest municipal bankruptcy in US history.

Revenue bonds are backed by the revenue generated from a specific project — toll roads, airports, water utilities, housing projects. If the project underperforms, bondholders have no claim on the issuer's general tax revenues. Revenue bonds typically offer slightly higher yields to compensate.

Private activity bonds fund projects with both public and private use (stadiums, airports used by private airlines). While interest is generally tax-exempt, it may trigger the alternative minimum tax (AMT) for some investors. Always check AMT status before purchasing.

The Taxable Equivalent Yield

The key metric for comparing muni bonds to taxable bonds is the taxable equivalent yield (TEY). The formula:

TEY = Tax-Exempt Yield / (1 − Marginal Tax Rate)

An example: A muni bond yields 3.5% and you are in the 35% federal bracket. The TEY is 3.5% / (1 − 0.35) = 5.38%. You would need a taxable bond yielding 5.38% to match the after-tax income from that muni.

If the muni is also exempt from your state income tax (say 6%), the calculation adjusts further:

TEY = 3.5% / (1 − 0.35 − 0.06) = 5.93%

This is why munis almost never make sense in IRAs or 401(k)s — you are already receiving tax deferral or tax-free growth in those accounts, so holding tax-exempt bonds there gives up yield for a benefit you cannot use. Munis belong in taxable brokerage accounts.

Credit Quality

Munis have a reputation for safety, and historically default rates are low — roughly 0.1% for investment-grade munis versus over 2% for similarly-rated corporate bonds over comparable periods. But headlines about pension shortfalls in Illinois, New Jersey, and other states are not theoretical. State and local governments carry enormous unfunded liabilities, and bondholders stand behind pension obligations in the priority of claims.

Stick to investment-grade munis (rated BBB/Baa or higher) from issuers with diverse tax bases and manageable debt loads. A single-state muni fund concentrates geographic risk — if you live in Illinois, a national muni fund may be worth the state tax cost for the diversification benefit.

How to Invest

Individual bonds offer known maturity dates and full return of principal at maturity if held and if the issuer does not default, but require meaningful diversification across issuers, states, and sectors — at least 10–15 bonds to be reasonably diversified. Transaction costs for retail investors can be opaque.

Muni bond mutual funds and ETFs provide instant diversification and professional credit analysis. The tradeoff is ongoing expense ratios. Major ETFs like MUB (iShares National Muni Bond ETF) charge 0.05%, making them efficient vehicles for most investors.

Muni money market funds hold short-term muni paper and maintain a stable $1 NAV. Yields fluctuate with short-term rates, making them useful as cash management vehicles for high earners.

Who Should Use Munis

Munis make the most sense for investors in the 32% federal bracket and above (taxable income over roughly $192,000 single / $384,000 married in 2025). Below that threshold, the after-tax yield advantage often disappears after adjusting for the lower yields munis offer relative to comparable taxable bonds.

They are particularly powerful for investors with substantial taxable accounts who have already maxed out tax-advantaged space — the classic case being a high-earning professional with a fully-funded 401(k) and backdoor Roth IRA, sitting on a seven-figure taxable brokerage balance. For someone in that position, swapping a portion of the bond allocation from a total bond fund to an intermediate muni fund can add 50–80 basis points of after-tax yield with no change in risk profile.