What Is the Difference Between Collateralized Debt Obligations (CDOs) and Mortgage-Backed Securities (MBS)?

A mortgage-backed security (MBS) is paid from a pool of home loans. A collateralized debt obligation (CDO) is paid from a pool of other debts, such as bonds, business loans, or slices of other securities, and it splits that cash into tranches that take losses in a set order. Before 2008, many CDOs were built from the lower-rated tranches of private-label MBS (securities issued by private firms without an agency guarantee), which is why the two are often discussed together. Today the same structure is used for collateralized loan obligations (CLOs), which the Federal Reserve describes as predominantly backed by leveraged corporate loans, not mortgages.

For the basics of how mortgage pools become securities, see what a mortgage-backed security (MBS) is.

What is a collateralized debt obligation (CDO)?

A CDO is a structured security: it buys a pool of debt and pays for it by selling its own securities to investors. A CDO manager, working with a securities firm, picks the assets. The Financial Crisis Inquiry Commission (FCIC), the panel Congress set up to investigate the 2008 crisis, describes the pre-crisis version as an instrument that buys and pools financial assets "such as the riskier tranches of various mortgage-backed securities."

The CDO then sells tranches, slices with different claims on the cash coming in:

• Senior tranches are paid first and carry the highest credit ratings, so they pay the lowest yields.
• Mezzanine tranches sit in the middle, with ratings such as AA, A, or BBB, and take losses after the junior layer runs out.
• The equity tranche is paid last and absorbs the first losses. It is unrated and gets whatever cash remains, so it has the highest potential return.

This payment order is called the waterfall.

A worked example of the loss order

Take a hypothetical $100 million CDO with an $80 million senior tranche, a $15 million mezzanine tranche, and a $5 million equity tranche.

The senior investors are protected until losses pass $20 million ($5 million + $15 million). That threshold applies whether defaults occur together or at different times.

CDO vs. MBS: how they compare

The biggest difference for an investor is the guarantee. With an agency MBS, the Federal Reserve Bank of New York explains, mortgage credit risk is borne by the guarantor, not the investor. The guarantee covers principal and interest payments, not market value. Interest-rate changes can lower the price, so you can lose money if you sell. CDOs lack government or agency guarantees, though some tranches have had private guarantees or other credit protection. Losses on the underlying debt can reach every tranche, including senior tranches once junior protection is exhausted.

Our page on residential mortgage-backed securities (RMBS) covers agency and private-label MBS in more detail.

How is a CDO different from a CMO?

A collateralized mortgage obligation (CMO) is a type of MBS, not a CDO. It holds mortgages or mortgage securities and splits their payments into tranches by a priority of payments, according to the SEC. Most are set up as real estate mortgage investment conduits (REMICs), a tax structure that keeps the trust itself from being taxed.

The tranches are split for a different reason:

• Agency CMOs (guaranteed by Ginnie Mae, Fannie Mae, or Freddie Mac) divide cash by prepayment timing. Per the FHFA, the classes differ in how sensitive they are to prepayments and in their average lives and final maturities, so investors choose how much early-payoff risk to take. Default risk stays with the guarantor.
• CDOs and private-label MBS divide cash by credit risk. Losses go to the most junior tranche first.

How CDOs spread losses in 2008

Before the crisis, mezzanine tranches of private-label MBS, mostly rated BBB or A, were hard to sell. Securities firms bought them and repackaged them into new CDOs. The FCIC reported that:

1. About 80% of these CDO tranches were rated AAA, even though the collateral was mostly lower-rated MBS tranches.
2. CDOs became the main buyer of risky MBS. By 2004, CDO creators were the dominant buyers of BBB-rated MBS tranches, and by 2005 they were buying "virtually all" of them. MBS made up more than half of CDO collateral by 2004, up from 35% in 2002.
3. Volume grew fast. Sales of these CDOs jumped from $30 billion in 2003 to $225 billion in 2006, and Wall Street issued nearly $700 billion of CDOs with MBS as collateral from 2003 to 2007.
4. Layers were added. CDOs with 80% to 100% of their cash in other CDOs were known as "CDOs squared." Synthetic CDOs held no mortgages at all, only credit default swaps that paid off based on how mortgage securities performed.

The rating agencies assumed that pooling many BBB tranches reduced the likelihood of losses reaching senior tranches. When home prices fell nationwide, the MBS tranches inside these CDOs defaulted at roughly the same time. The FCIC found that 20% of U.S. CDO securities were downgraded in 2007 and 91% in 2008. Among the 2006-issued CDOs backed by asset-backed securities that the FCIC analyzed, more than 80% of bonds first rated Aaa by Moody's were eventually cut to junk. The FCIC found that much of the loss fell on a small group of large financial companies holding the highest-priority and other AAA tranches that were supposed to be safest. For the full sequence, see what caused the 2008 mortgage crisis.

What the CDO market looks like now

The CDO structure is still in use in collateralized loan obligations. In its May 2026 Financial Stability Report, the Federal Reserve reports that non-agency securitization, including CDOs and CLOs, stayed strong through March 2026.

A leveraged loan is a loan to a company that already carries heavy debt or has a low credit rating. The Congressional Research Service reported in 2019 that more than 60% of leveraged loans were packaged into CLOs, with senior tranches held mostly by banks and insurers and junior tranches mostly by hedge funds and other asset managers. The loss order works the same way as in the example above, but the pool holds business loans instead of mortgage securities.

That means a CLO's risk depends on corporate defaults, not home prices. The CRS noted that leveraged loan default rates rose from below 1% to almost 11% in the 2007 to 2009 recession, which shows how much stress the junior tranches can face in a downturn.

To see which mortgage securities individual investors can buy, compare specific examples of mortgage-backed securities with the main ways to earn income from other people's mortgages.