Structured Credit Investing: ABS, CMBS, RMBS, CLOs, and How Losses Move Through the Tranches

Structured credit is the market for bonds carved out of loan pools. A lender or asset manager moves loans into a separate legal entity, which issues several classes of debt and a slice of equity against them. The same structure sits behind a bond backed by car loans, a bond backed by office mortgages, and a AAA tranche of a collateralized loan obligation (CLO). What differs is the collateral, and that difference explains most of what went right and wrong in 2008, in 2020, and in commercial property since 2023.

This guide covers the main sectors, how a waterfall moves losses from the bottom up, what the default record shows, and how individual investors can get exposure. Our CLO investing guide goes deeper on CLO structure, manager selection, and the stress CLO ETFs went through in April 2025. Structured notes, which are bank debt with option payoffs rather than pools of loans, are covered in our structured notes guide.

The four main sectors

CDOs, which pooled other securitized bonds (often the junior tranches of subprime RMBS), were the sector that failed hardest in 2008. They are largely gone from the new-issue market.

How losses move through a waterfall

Each deal has two waterfalls. Interest goes to fees and then to each tranche's coupon in order of seniority, with anything left over going to equity. Losses on the loans run the other way: they reduce the pool's par value and are absorbed by equity first, then the most junior debt, and so on up.

Illustration: a $100 million pool funded by a $75 million senior tranche, a $15 million mezzanine tranche, and $10 million of equity. Loss equals the share of loans that default times one minus the recovery rate. The numbers are ours and the structure is simplified, with no excess interest used to absorb losses.

At 60% recovery, the mezzanine is untouched until 25% of the pool defaults and the senior tranche until 62.5%. At 30% recovery, those thresholds fall to 14.3% and 35.7%. Recovery assumptions matter as much as default assumptions, and loan recoveries have fallen in recent years, as the CLO guide describes.

Coverage tests redirect cash before losses arrive

CLOs and many other deals add overcollateralization (OC) and interest coverage (IC) tests. Continuing the illustration: pool par divided by the senior and mezzanine debt starts at 1.111. Suppose the mezzanine OC trigger is 1.05. A $6 million par loss drops the ratio to 1.044, and the test fails.

In a year when the pool earns 8% and the tranches cost 5.5% (senior) and 8.5% (mezzanine), with fees of 0.5%, the equity would normally receive $2.10 million, a 21% cash yield on $10 million. After the $6 million loss, the pool earns $7.52 million and about $1.65 million is left for equity. Of that, about $0.48 million must go to pay down the senior tranche to bring the ratio back to 1.05, so equity receives about $1.17 million. Senior investors are paid down faster exactly when the pool weakens, which is why AAA tranches held up in 2008 and 2020 while equity payouts were cut.

What the default record shows

S&P Global Ratings has published annual default and rating transition studies for structured finance for decades. On its investor factbook page, S&P shows a five-year cumulative default rate of 3.49% for AAA structured finance issues, against 0.34% for AAA corporate issuers.

Old mortgage bonds still dominate the default counts. In S&P's 2024 structured finance study, legacy US RMBS issued before 2009, mostly subprime and alt-A bonds already rated CC, accounted for 93% of the year's defaults. US RMBS had a 6.9% annual default rate against a one-year average of 4.7%. US CMBS had the highest downgrade rate of any sector, 16.1%, while ABS had the highest upgrade rate, 13.5%.

CLOs have a different record. According to S&P data summarized in our CLO guide, no AAA CLO tranche has ever defaulted, and of 4,322 ratings on CLOs issued before 2010, 40 defaulted. The reasons are structural: CLOs hold senior secured corporate loans rather than other securitized bonds, the loans are spread across many companies and industries, and the coverage tests are based on par and ratings rather than market prices.

2008 and 2020 for CLOs

Larry Cordell, Michael Roberts, and Michael Schwert studied CLO cash flows in a Philadelphia Fed working paper (revised November 2021). CLOs issued in 2006 and 2007 had locked in cheap long-term financing before the crisis, and their managers reinvested in high-yielding loans during and after it. The authors describe the result as a windfall of excess interest and principal for CLO equity investors as the economy recovered. They found the COVID shock of 2020 had, up to the time of their analysis, a negligible effect on equity distributions.

They attribute that resilience to a few features: CLOs are closed-end vehicles, so investors cannot force sales; coverage tests use ratings and par rather than market prices, so price swings alone do not divert cash; managers can reinvest and refinance; and the debt is locked in for the life of the deal. The paper also found that the loans in CLO pools returned about the same as a broad leveraged loan index before fees, so the average manager showed no skill at picking loans, although some managers did better persistently.

The comparison with CMBS since 2023 is useful. CMBS pools can be concentrated in a few large loans and one property type, and borrowers must refinance at maturity. Trepp's August 2026 figures, as reported by Connect CRE, showed about $47.4 billion of CMBS loans more than 30 days late, with office loans making up just over 42% of the delinquent balance. A single large office loan can sit in several deals, so read the top ten loans in any CMBS deal you consider.

How individuals get access

CLO tranches and many other deals are sold under Rule 144A to qualified institutional buyers in large minimum sizes, so individuals usually invest through funds.

Terms of the two largest AAA CLO ETFs, from the managers' own pages:

Sources: JAAA fund page; CLOA fact sheet dated June 30, 2026, the most recent we could retrieve. CLOA's net asset value returned 7.31% in 2024 and 5.58% in 2025, according to the same fact sheet.

Because the coupons float, these funds' yields fall when short-term rates fall, and a AAA CLO fund behaves more like a floating-rate cash alternative with credit spread risk than like a bond fund. Their distributions are taxed as ordinary income.

Interval funds sit at the other end. FINRA's Regulatory Notice 22-08 lists them among complex products and notes that they are generally required to repurchase only 5% to 25% of their shares, usually quarterly. A fund that holds CLO equity can pay a high distribution while its net asset value falls, so compare total return, not the distribution rate.

Which layer suits which investor

• AAA CLO ETFs: cash-like holdings for investors who accept small price dips in credit stress in exchange for a yield above Treasury bills.
• Investment grade mezzanine (A and BBB): investors who can tolerate larger drawdowns and understand that these tranches sit closer to the loss line when recoveries are low.
• Below investment grade tranches and equity: investors who can hold through a full credit cycle, accept that distributions can stop when coverage tests fail, and pay attention to manager selection. Our private credit guide and high yield bond guide cover the alternatives for the same risk budget.
• Agency MBS: investors who want high-quality bonds and accept that prepayments speed up when rates fall and slow down when rates rise.

Questions to ask before buying a structured credit fund

1. What sectors and rating levels does it hold, and how much sits below investment grade?
2. How much credit enhancement does the average tranche have, and what recovery rate does the manager assume?
3. How did the fund perform in March 2020 and April 2025, and did it meet redemptions in cash or in kind?
4. For CMBS exposure: how much office, and how concentrated are the largest loans?
5. For interval or closed-end funds: what are the total expenses including interest on borrowing, and what share of recent distributions came from net investment income?
6. For CLO funds: which managers' deals does it own, and how many deals are past their reinvestment period?

Our alternative investments guide covers how much of a portfolio to put in less liquid credit.

This guide is for informational purposes only and does not constitute investment, tax, or legal advice. Fund yields, assets, and fees are as of the dates shown in September 2026 or earlier and change frequently; read the current prospectus. Structured credit can lose value, and lower tranches and fund borrowing can magnify losses. Consult qualified professionals before investing.