Interest only strips are mortgage-backed securities built from just the interest portion of a pool of home loan payments, with the principal peeled off and sold separately. Their defining trait is backwards from a normal bond: they gain value when interest rates rise and lose value when rates fall. That inverted behavior comes almost entirely from how quickly homeowners prepay their mortgages, and it is what makes IO strips both a specialized hedging tool and one of the trickier instruments in fixed income.
How an IO Strip Is Built
Every mortgage payment contains two parts: interest and principal. Stripping takes the cash flow from a pool of securitized mortgages and separates those two streams into standalone securities. The IO strip receives every dollar of interest the borrowers pay. The principal only (PO) strip receives every dollar of principal, including scheduled amortization and any early payoffs. Neither piece touches the other’s cash.
Most IO strips come out of structured programs run by government-sponsored entities. Fannie Mae issues them through its Stripped Mortgage-Backed Securities (SMBS) program.1Fannie Mae. SMBS – Capital Markets Ginnie Mae runs a similar program and guarantees timely payment of principal and interest on each class of its stripped MBS.2Ginnie Mae. Stripped MBS IO strips also appear inside Real Estate Mortgage Investment Conduits (REMICs) as regular interests entitled to a specified portion of the interest on the REMIC’s mortgages.3Federal Register. Interest-Only REMIC Regular Interests
No Principal Comes Back
An IO strip has no principal that returns to you at maturity. It references a notional principal balance, which is the outstanding loan balance in the underlying pool used to calculate how much interest you receive each month. Your cash flow equals the coupon rate multiplied by whatever notional balance still exists. A REMIC IO can carry a specified principal amount of zero; the security entitles you to a share of the interest and nothing else.3Federal Register. Interest-Only REMIC Regular Interests
Because the entire return comes from interest on a shrinking balance, the security can produce a negative total return if that balance disappears too fast. There is no par value waiting at the end to make you whole.
Prepayment Is the Whole Story
Each month, borrowers in the pool make their payments and you receive the interest portion. As they amortize their loans, the pool’s outstanding balance shrinks and your interest payments shrink with it. That decline is predictable and priced in from the day you buy.
What is not predictable is prepayment. When homeowners sell, refinance, or pay extra toward the loan, the remaining principal on those mortgages disappears immediately. Every dollar of principal that vanishes early is a dollar that will never generate another interest payment for you. Fast prepayments are the central risk to an IO holder. A refinancing wave can wipe out large chunks of the notional balance and cut the interest stream you paid for short.
Prepayment speeds get modeled against standard benchmarks so buyers and sellers can talk about the same scenario. The price of an IO strip reflects the market’s current view of which speed will play out. Get that view wrong and the security’s return moves sharply.
Why the Price Rises When Rates Rise
A traditional bond falls in value when interest rates climb, because its fixed coupon looks worse next to newly issued bonds paying more. IO strips do the opposite, and the reason is borrower behavior.
When market rates fall, homeowners rush to refinance their higher-rate mortgages. That is a mass prepayment event. The principal balance shrinks rapidly, future interest payments vanish, and the IO strip’s value drops. You paid for years of interest that just got truncated.
When rates rise, borrowers have no reason to refinance a low-rate loan into a higher-rate one. They stay put. Prepayments slow to a trickle, the principal balance lasts longer than the market had priced in, and the IO strip throws off more total interest than expected. The value goes up.
This is called negative duration. A normal bond has positive duration and loses value when rates rise; an IO strip has negative duration and gains value when rates rise. That property is why institutions holding large portfolios of conventional bonds use IO strips as a hedge: losses on the traditional side can be offset by gains on the IO position.
The Asymmetry: Negative Convexity
The price-rate relationship is not a clean straight line. When rates fall past the point where refinancing saves a borrower roughly 50 basis points, prepayment speeds can jump from single digits to well above 40 percent. The IO strip’s value falls much harder than a simple duration figure would suggest.
On the other side, when rates are already high, prepayments settle into a narrow range of about 4 to 9 percent, driven mostly by home sales and cash-out activity. Further rate increases produce diminishing benefit for the IO holder because prepayments are already about as slow as they get. The result is negative convexity: the downside when rates fall is steeper than the upside when rates rise by the same amount. Misjudging a rate decline costs more than misjudging a rate increase by the same margin.
Credit Risk Depends on the Issuer
For agency-backed IO strips through Ginnie Mae, Fannie Mae, or Freddie Mac, credit risk on the underlying mortgages is largely removed. The agencies guarantee timely payment of principal and interest on the underlying MBS, and the risk of borrower default sits with the guaranteeing agency rather than the investor.2Ginnie Mae. Stripped MBS4Federal Reserve Bank of New York. Understanding Mortgage Spreads
Defaults still reach the IO holder indirectly. When a defaulted loan is liquidated, its principal is retired, which functions like a prepayment: the interest from that loan ends. Non-agency IO strips, backed by private-label MBS without a government guarantee, carry full credit risk on top of prepayment risk and are considerably more volatile.
Who Actually Owns These
IO strips are overwhelmingly institutional. Banks, insurance companies, mortgage servicers, hedge funds, and specialized fixed-income managers make up the bulk of the market. Mortgage servicers pay particular attention because mortgage servicing rights share the same risk profile: both generate income tied to the outstanding balance of a mortgage pool, both gain value when rates rise and prepayments slow, and both lose value when refinancing surges.
Retail access exists in principle but is limited in practice. A broker recommending an IO strip has to satisfy FINRA’s suitability rule, which requires a reasonable basis for believing the investment fits the customer’s profile, including risk tolerance, experience, time horizon, and liquidity needs.5FINRA. FINRA Rule 2111 (Suitability) FAQ Given the complexity and the potential for negative returns, most firms restrict these products to institutional or high-net-worth accounts. Individual investors who want exposure usually get it through mutual funds or ETFs that hold IO strips inside a broader mortgage strategy rather than buying strips directly.
Tax Treatment and Phantom Income
IO strip income is taxed as ordinary income, not capital gains. Under the Internal Revenue Code, buying a stripped coupon is treated as if you bought a bond originally issued at a discount, and the gap between what you paid and the total payments you expect to receive is classified as original issue discount (OID).6Office of the Law Revision Counsel. 26 USC 1286 – Tax Treatment of Stripped Bonds You include that OID in gross income over the life of the security on a constant-yield basis, not when cash actually arrives.7Office of the Law Revision Counsel. 26 USC 1272 – Current Inclusion in Income of Original Issue Discount
That accrual method creates a problem investors call phantom income. Because the code requires you to recognize OID by formula rather than by cash received, there will be periods when your taxable income exceeds the cash in your account. You owe tax on income you haven’t collected. The gap tends to be worst early in the security’s life, or when a shift in prepayment speeds pulls the expected cash flow pattern out of shape. Buying at a premium lets you amortize that premium against reported income; buying at a discount makes the accrual larger.
Issuers report OID on Form 1099-OID, and the IRS publishes tables and guidance for calculating it in Publication 1212.8Internal Revenue Service. Instructions for Forms 1099-INT and 1099-OID9Internal Revenue Service. Publication 1212 – Guide to Original Issue Discount (OID) Instruments Most holders rely on their custodian’s tax reporting or dedicated software, because tracking OID by hand across a mortgage pool with shifting prepayment speeds is not a workable exercise.