Municipal Bonds Explained: Tax-Free Income and Its Fine Print

The exemption everyone buys these bonds for is not a constitutional right. For most of the twentieth century it was widely assumed to be one, on the reasoning that a federal tax on the interest paid by a state would be a tax on the state itself and therefore off limits. The Supreme Court settled the question the other way in 1988, in South Carolina v. Baker. It held that nothing in the Constitution prevents Congress from taxing the interest on state and local bonds. The exemption survived because it lives in the Internal Revenue Code. Congress put it there. Congress can reach it.

That is the correct frame for the whole asset class, because a municipal bond is a loan to a state, a city, a school district or an authority that runs something like a toll road, and what makes it a separate market is a provision of the tax code. Congress has narrowed the provision before, most visibly in 1986 when it made interest on certain private activity bonds a preference item for the alternative minimum tax, and again in 2017 when it withdrew the exemption from advance refundings. The quoted yield needs an arithmetic step before it is comparable with anything else. That arithmetic rests on a statute.

Two promises, and they are not the same promise

Municipal debt divides into two families. The difference is what stands behind the payment.

General obligation bonds are backed by the full faith, credit and taxing power of the issuer, and if revenues fall short, the issuer has the legal authority to raise taxes and pay you. Some general obligation bonds carry an unlimited tax pledge and others a levy capped at a stated rate, a distinction that appears in the offering document and almost nowhere else, and which decides what the pledge is worth in the one situation where the pledge is tested.

Revenue bonds are backed by the income of a single enterprise: an airport, a water utility, a hospital, a stadium. Nothing else stands behind them. If traffic on the toll road comes in below the projections printed in the prospectus, the bondholders absorb the difference, and no general tax revenue is under any obligation to close the gap.

Revenue bonds make up the larger share of the market. They carry by far the wider range of credit quality. A municipal water system with a captive customer base and a hospital competing for patients in a town that is shrinking are both revenue credits, and they have almost nothing in common beyond the label.

The comparison that has to be made first

The formula is short. It decides everything:

Tax-equivalent yield = municipal yield / (1 - marginal tax rate)

Take a muni yielding 3.40% and an investor in a 32% federal bracket:

1. The after-tax fraction of any taxable yield is 1 - 0.32 = 0.68.
2. 3.40% / 0.68 = 5.00%.

That investor would need a taxable bond yielding 5.00%. That is what keeps the income the same after tax. Turn it around and the point is sharper: a taxable corporate at 4.60% leaves 4.60% x 0.68 = 3.13% after federal tax, which is less than the 3.40% the muni pays free of it, so the corporate bond that looks better on a screen is the worse holding for this particular person.

The same bond, six different investments

The answer depends entirely on the bracket, which is why the same bond is a sound holding for one buyer and a poor one for the buyer sitting next to them. The marginal rates below are illustrative. The schedule that applies to you is published by the IRS.

Marginal federal rate	Tax-equivalent yield on a 3.40% muni
12%	3.86%
22%	4.36%
24%	4.47%
32%	5.00%
35%	5.23%
37%	5.40%

The breakeven is worth knowing from the other direction as well. Set it against a taxable bond yielding 4.50%. The muni comes out ahead for anyone whose marginal rate exceeds 1 - (3.40 / 4.50) = 24.4%, and behind for everyone below it. Municipal bonds are priced by a market dominated by buyers in the higher brackets, which is precisely why the pre-tax yield looks unattractive to everybody else. The discount is already in the price.

Three adjustments that push it further

The 3.8% net investment income tax applies to taxable interest and leaves municipal interest alone, so an investor in a 32% bracket who is subject to it compares at 3.40% / (1 - 0.358) = 5.30%. The income thresholds at which that tax begins are set by statute. Check them with the IRS.

An in-state bond usually escapes state income tax as well, so an investor facing a 5% state rate on top of a 32% federal rate compares at 3.40% / 0.63 = 5.40%. This is the argument for single-state funds, and it is also the argument against them, since concentrating credit exposure in the state whose economy already pays your salary is a peculiar way to manage risk.

The third adjustment runs the other way. Interest on some private activity bonds is a preference item for the alternative minimum tax, a consequence of the 1986 reform mentioned above, and for an investor who falls into that calculation the exemption is partly undone. Which category a bond belongs to is stated in the offering document. Look in the section headed tax matters, where it has been all along.

Credit risk, rare and real

Rated general obligation issuers have defaulted far less often than corporate borrowers carrying comparable ratings, which is what you would expect of borrowers who can levy taxes, and that record supports a narrower credit spread on munis than on corporates. It does not support treating them as free of credit risk. The episodes that prove the point turn on documents.

In 1983 the Washington Public Power Supply System defaulted on roughly $2.25 billion of revenue bonds issued to build two nuclear plants that were never finished. The bonds had been backed by take-or-pay contracts under which participating utilities agreed to pay whether or not any power was ever delivered, and the Washington Supreme Court then held that the utilities had lacked the authority to enter those contracts in the first place. The security the bondholders believed they held was struck down. It was the largest municipal default up to that time. Nothing about the economy caused it. A court read the enabling statute and the pledge evaporated.

Detroit entered Chapter 9 bankruptcy in 2013, and Puerto Rico worked through a lengthy restructuring in which holders of bonds widely regarded as secure took losses. Both came out of long-building pension and revenue problems. The audited financial statements had shown them years earlier.

The warning signs are the ones a credit analyst looks for in any borrower: pension funding levels, a shrinking tax base, repeated use of one-time revenues to close operating gaps, and dependence on a single large employer, and the rating scales and the spread mechanics work exactly as they do in corporate bonds and credit spreads.

The market itself is the harder problem

There are far more municipal issuers than corporate ones. Most individual bonds trade rarely or never. You feel the consequences in the price.

Dealer markups are embedded in the quoted price. There is no commission line to point at. On a small lot the round trip can consume a meaningful share of a year’s interest, which means a muni bought and sold inside a year can lose to a Treasury bought at auction for nothing at all.

Most munis are callable, conventionally about ten years after issue, so a fall in rates takes away the bond you wanted to keep at precisely the point where its income had become most valuable. Price that with yield to worst. It is explained in bond yields explained and computed by the bond yield calculator.

The de minimis rule, which catches careful buyers

One more provision catches people who have done everything else correctly. Buy a muni at a discount larger than a quarter of a point of face value for each remaining year, and the accretion back to par is taxed as ordinary income. After a period of rising rates, older low coupon munis fall into that territory in large numbers, and their prices have to drop further still to compensate a buyer for the treatment, which is a second price fall layered on top of the first.

Interest rate risk, meanwhile, is identical to that of any other bond, and a long muni with a modified duration of 8 loses roughly 8% of its value when yields rise a point, exactly as set out in duration and interest rate risk. The tax treatment of the income changes nothing about the arithmetic of the price.

Where munis fit

Municipal bonds are built for taxable accounts belonging to investors in the higher brackets. Inside an IRA or a 401(k) the income is already sheltered, so accepting a lower yield to avoid a tax you would never have paid gives up return for nothing. That placement question is the subject of tax-efficient investing.

For most people the practical vehicle is a fund, which spreads several hundred issuers and absorbs the trading costs at institutional scale, at the cost of the maturity date an individual bond gives you. See bond ETFs explained for those mechanics. Compare what the same maturity pays with no credit risk at all in the Treasury securities guide.

Before deciding that any of these yields is adequate, run it through the real return calculator to see what survives inflation, which is a tax nobody legislated and nobody exempts you from. If the underlying price and yield arithmetic needs a refresher, how bonds work has it, and the bonds and yields quiz will tell you what stuck.