The municipal bond market is the one corner of fixed income where a forced seller can look for a buyer and find almost nobody there. That is not a flaw in the plumbing. It follows directly from who owns these bonds and why. The clearest demonstration came in March 2020, when the safest tax-exempt debt in America briefly yielded roughly three times what the US Treasury paid.
Three Weeks in March 2020
In the first half of March 2020, Treasuries rallied as investors fled to safety, which is the textbook response. Munis did the opposite. Over roughly two weeks, yields on high-grade municipal bonds rose by more than a full percentage point, while comparable Treasury yields fell.
By widely reported measures, the muni/Treasury ratio went above 300% at the 10-year point at the peak. This ratio is the muni yield divided by the Treasury yield of the same maturity. The muni pays no federal income tax and the Treasury does, so the ratio normally sits below 100%.
At 300%, a tax-exempt AAA school district bond was paying triple the yield of a taxable Treasury. No tax-based reasoning can justify that. A pension fund paying zero tax would have been better off owning the muni.
Mutual funds and ETFs were what drove the move. Muni funds saw record weekly outflows that month, and to meet redemptions they had to sell bonds into a market where dealers had little balance sheet to spare. The Federal Reserve announced its Municipal Liquidity Facility on April 9, 2020, a lending backstop for state and local issuers. Even then, ratios took months to settle back toward normal, long after equities and investment-grade corporate credit had bounced.
Most observers saw a panic that eventually passed. The more useful observation is the gap in recovery speed. Every market sold off in March 2020. The muni market was simply the slowest to heal, and the reasons are structural.
Who Actually Owns a Muni, and Why That Changes Everything
A municipal bond is debt issued by a state, city, county, school district, water authority, hospital system, airport, or any of thousands of other public entities. The defining feature is that interest on most munis is exempt from federal income tax, and often from state tax for residents of the issuing state.
That exemption decides who the buyer is. The tax break is worth the most to people in the highest tax brackets, so the natural owner is a wealthy individual, either directly or through a mutual fund, ETF, or separately managed account. Banks and property and casualty insurers make up most of the rest.
These owners behave very differently from the buyers of Treasuries or corporate bonds. A retiree holding a 20-year California general obligation bond bought it for tax-free income. They are not marking the position to market daily, and they have no reason to sell when a rival bond gets cheaper. Most hold to maturity.
The supply side is just as fragmented. There are tens of thousands of issuers and roughly a million distinct securities outstanding. A single city might have dozens of separate bond issues, each with its own maturity, coupon, call schedule, and credit backing. Compare that with the Treasury market, where a few hundred securities from one issuer trade trillions of dollars a day.
The result is a market where the vast majority of outstanding bonds do not trade on any given day. Pricing for an untraded bond is an estimate, built by matrix models from bonds that look similar.
What this means for you: the price on your muni statement is often a model's opinion, not the price you would get if you sold that day. In calm markets the difference is small. In a stressed market it can be large.
The Arithmetic of Tax-Exempt Yield
Muni yields only make sense through the tax-equivalent yield, which is the taxable yield you would need to earn the same after-tax income.
Tax-equivalent yield = Muni yield / (1 - Tax rate)
Take a muni yielding 3.5%, held by an investor in the top 37% federal bracket who also owes the 3.8% net investment income tax, for a combined rate of 40.8%.
Tax-equivalent yield = 3.5% / (1 - 0.408) = 3.5% / 0.592 = 5.91%
That investor would need a taxable bond yielding almost 5.9% to match the tax-free 3.5%. A state resident who also avoids a high state income tax would need even more.
The same math gives a breakeven tax rate from the muni/Treasury ratio:
Breakeven tax rate = 1 - (Muni yield / Treasury yield)
If a 10-year muni yields 3.0% and the 10-year Treasury yields 4.0%, the ratio is 75%. The breakeven tax rate is 1 - 0.75 = 25%. Anyone with a marginal rate above 25% prefers the muni, and anyone below it prefers the Treasury.
Now plug in March 2020. At a 300% ratio, the breakeven tax rate is 1 - 3.0 = -200%. A negative breakeven means the muni beat the Treasury even for an investor who pays no tax at all. On tax logic alone, the bond was mispriced for every investor in the world.
What this means for you: the ratio is the cleanest single read on whether munis are cheap or rich. Below about 70% at 10 years, you are paying up for the tax break. Above 100%, you are being paid to own tax-exempt debt that a fully taxable investor would also want.
Why Arbitrage Capital Arrives Late
This is where intuition goes wrong. Most readers assume that a bond yielding three times its fair value gets bought within hours, because that is how Treasuries, large-cap stocks, and currencies work. In munis, the capital that fixes mispricings is small, slow, and poorly equipped.
Start with the natural buyer. The taxpaying individual who values the exemption most is not watching ratios intraday and usually cannot act quickly. Many hold munis through funds that were themselves being redeemed, so this group was a net seller in March 2020, not a buyer.
The next group is crossover buyers: insurers, hedge funds, taxable bond funds, and some foreign investors who buy munis only when they are cheap enough to beat taxable alternatives without any tax benefit. They do arrive, but they need a large discount because the exemption is worth little or nothing to them. They also have to do credit work on unfamiliar issuers, and they face the same thin dealer liquidity when they eventually want out.
Then there is the hedging problem. In most markets, an arbitrageur buys the cheap asset and shorts the expensive one. Munis cannot practically be shorted in size. There is no deep repo market for borrowing them, and with a million different CUSIPs there is no liquid benchmark bond to sell against. A buyer of cheap munis has to hedge with Treasury futures or interest rate swaps, which offsets rate risk but leaves the muni/Treasury ratio itself unhedged. If the ratio keeps widening, the hedged buyer still loses.
Finally there is a timing mismatch built into the funds. Muni ETFs and mutual funds offer daily liquidity on top of bonds that may not trade for weeks. When redemptions spike, the fund has to sell whatever is most liquid, which pushes down the prices of the best bonds first. Selling begets lower marks, lower marks prompt more redemptions, and the loop runs until outside capital or a central bank breaks it.
What this means for you: a muni dislocation lasts as long as it takes slow, unhedgeable capital to get comfortable. That is weeks to months, not hours.
Where the Mechanism Breaks
The buy-and-hold, tax-driven structure is durable, but it is not permanent. Three forces can override it.
The first is outside intervention. The Municipal Liquidity Facility in April 2020 lent relatively little in dollar terms, but its existence capped the tail risk for issuers and gave crossover buyers the confidence to step in. A credible backstop can shorten a dislocation that the market itself would have dragged out.
The second is a change in the tax code, which reshapes the demand base itself. The 2017 Tax Cuts and Jobs Act cut the corporate tax rate from 35% to 21%, which made the exemption less valuable to banks and insurers, two of the largest institutional holders. It also ended tax-exempt advance refundings, a technique issuers had used to refinance outstanding bonds early. Any future change to individual rates or the exemption would move the breakeven math for the core buyer overnight.
The third is genuine credit failure, where "slow to heal" becomes "does not heal." Detroit's 2013 bankruptcy and Puerto Rico's debt crisis, which led to a 2016 default and years of restructuring, were not liquidity dislocations that a patient buyer could wait out. Those bonds were repriced because recovery values were truly in doubt. The ratio framework assumes default risk is low, which holds for most of the market and fails badly for a stressed issuer.
Telling these cases apart is the real skill. March 2020 was a liquidity event dressed up as a credit scare, since high-grade issuers kept paying. Puerto Rico was a credit event, and waiting for mean reversion would have meant years of losses.
What this means for you: before treating a muni selloff as a buying opportunity, ask whether the issuers are being sold because of who holds them or because of what they owe.
Reading the Muni Market Today
The same structure shows up in calmer markets as a smoother, slower price path. A broad muni fund carries much shorter duration than a long-maturity Treasury fund, and its holders rarely sell on rate moves, so day-to-day swings are smaller. That smoothness is partly real and partly a product of model-based pricing on bonds that do not trade.
Stress breaks the smoothness rather than easing into it. When muni funds face sustained outflows, as they did during the 2022 rate selloff, prices can fall sharply and stay down even after Treasuries stabilize, because the sellers are funds and the patient buyers have not yet shown up.
The calendar also matters in a way that is unusual for bond markets. Large coupon and principal payments arrive in certain months, often cited as June, July, December, and January. In those months, holders reinvest cash into a market with limited new supply, which tends to firm up prices. It is a predictable flow that exists only because the owner base is so passive.
What this means for you: a muni fund's low volatility in normal times is no promise of low volatility in a crisis. Expect calm, then a sharp gap, then a slow recovery.
What a Holder Should Do Differently
Treat muni liquidity as conditional. Your bonds are easy to sell when nobody else is selling and expensive to sell when everyone is. Holding individual high-grade munis to maturity largely sidesteps this, because a holder who does not sell is not forced to accept a stressed price. Holding munis through a daily-liquidity fund puts you beside other holders who may need to sell at the worst moment.
Use the muni/Treasury ratio as your main reading tool. When it rises well above 100% for high-grade bonds, the market is offering compensation for illiquidity, not a warning about credit, unless the issuers involved actually have credit problems. Patient, after-tax buyers were paid handsomely for stepping in during March 2020, and that payment existed because the arbitrage capital that would normally have taken it was slow.
The mechanism fits in one sentence: tax-driven, buy-and-hold owners and unhedgeable, fragmented supply leave the muni market with few natural buyers in a crisis. That is why its dislocations last longest, and why patience is worth the most there.
