He Bought an 8% Municipal Bond for Tax-Free Income. The IRS Used It to Tax More of His Social Security.

Municipal bond interest is excluded from federal tax but counts in the IRS formula that can make up to 85% of Social Security benefits taxable.

Single filers earning above $34,000 and married couples above $44,000 face peak Social Security taxation, and those thresholds haven't changed in decades.

An 8% muni bond coupon may signal weak credit ratings, low liquidity, or single-source revenue risk. Being tax-free does not mean being risk-free.

Picture a retiree in his early seventies living on Social Security and a modest portfolio. He spots a municipal bond offering an 8% tax-exempt yield. With the 10-year Treasury paying roughly 4.7%, the choice looks almost too easy. The income is federally tax-free, the payout is generous and he can leave his principal invested.

Then his tax preparer runs the return. More of his Social Security is taxable than the year before. His benefit did not grow enough to explain it. The culprit is the municipal bond he bought precisely to avoid federal taxes.

Municipal bond interest is generally exempt from federal income tax. The IRS does not tax the interest itself, but it still includes those dollars when calculating how much of a retiree's Social Security is taxable. That calculation adds adjusted gross income (AGI), tax-exempt interest and half of annual Social Security benefits. Once the total crosses $25,000 for a single filer or $32,000 for a married couple filing jointly, up to 50% of benefits can become taxable. Above $34,000 and $44,000, respectively, as much as 85% can become taxable.

Those thresholds have not moved in decades. That leaves plenty of retirees close enough to the line for a new stream of municipal bond income to push them across it. The bond interest remains tax-exempt. It simply causes more of another income source to become taxable. That is the tax torpedo hiding behind the attractive yield.

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This has nothing to do with the retirement earnings test. That rule applies to wages and net self-employment income earned before full retirement age (FRA). Interest does not count, whether it comes from a municipal bond, Treasury or savings account. Social Security will not withhold his monthly checks. The surprise arrives on his tax return instead, when a larger portion of those checks becomes taxable.