The mechanism, plainly
A TIPS pays you two ways: a fixed coupon on an inflation-adjusted principal, and the principal adjustment itself, which accrues with CPI and is paid out when the bond matures. The tax code treats that annual principal accrual as original issue discount (OID): reportable, taxable interest income in the year it happens, even though not a dollar of it reaches your account until maturity. Your brokerage reports it to you (and the IRS) on Form 1099-OID; the coupon interest arrives separately on 1099-INT. The IRS's treatment is laid out in Publication 550's rules for inflation-indexed debt instruments.
A worked example
Say you hold a $10,000 TIPS and CPI inflation runs 3% this year:
Principal adjustment: $10,000 × 3% = $300 — taxable now, paid at maturity
Coupon (say 2.4% real): ~$247 on the adjusted principal — taxable now, paid now
Cash received this year: ~$247 · Taxable income this year: ~$547
That $300 gap between what you owe tax on and what you received in cash is the "phantom income." At a 24% marginal rate it is $72 of tax paid out of pocket this year for money that arrives at maturity. Multiply by a $500,000 ladder in a high-inflation year and the cash-flow mismatch stops being a curiosity: 3% inflation on $500,000 is $15,000 of phantom income — roughly $3,600 of current-year federal tax at 24% with no matching cash.
The placement rule that follows
Hold TIPS ladders in tax-advantaged accounts — traditional IRA, Roth IRA, 401(k) — whenever you have the room. Inside them, the OID accrual is a non-event: no annual tax, no 1099-OID bookkeeping, no cash-flow mismatch. This is the near-universal recommendation for a reason, and the calculus only strengthens for larger ladders and higher tax brackets. In taxable accounts TIPS are not unusable — retirees in low brackets, or anyone whose ladder is small relative to income, may find the phantom tax trivial — but it is a recurring cost and a recurring chore that an IRA makes disappear.
Three softenings worth knowing
State and local tax: exempt. Like all Treasuries, TIPS interest and OID accruals are exempt from state and local income tax — a real advantage over CDs and corporate bonds in high-tax states, taxable account or not.
Deflation nets against you gently. In a deflationary year the principal adjustment is negative, and that negative OID generally offsets the year's coupon income on the same bond — the tax mirror of the accrual itself.
The accrual is real money. Phantom income is deferred cash, not fictional cash: everything taxed along the way is money you actually receive at maturity. The issue is timing and bookkeeping, never a tax on nothing.
What this means for ladder builders
Decide the account before you buy the bonds. The order of operations that saves the most grief: fill IRA/401(k) space with the ladder first; put any taxable-account rungs in the nearest maturities (less accrued adjustment left to tax, and the cash arrives soonest); and if a large taxable ladder is unavoidable, budget for the annual tax drag explicitly — our calculator's per-year table gives you the principal figures to estimate it from. The fund-vs-individual choice doesn't change any of this, by the way: TIPS funds distribute their inflation accruals as taxable dividends, so the tax arrives either way — a ladder just makes the timing visible.