Standard & Poor's Corp. downgraded $4.61 billion of collateralized debt obligations Monday for reasons including credit deterioration and recent ratings cuts on subprime residential mortgage-backed securities.

The rating agency lowered 10 tranches from seven cash-flow CDOs composed of asset-backed securities and three tranches from a synthetic CDO. Nine of the 13 tranches were already junk-rated, while the other four had been investment-grade until Monday's downgrades.

Four of the seven cash-flow CDOs are collateralized in large part by mezzanine tranches of residential mortgage-backed and other structured-finance securities. Two are collateralized mostly by triple-A-rated through A-rated residential mortgage-backed and other securities. The other is a repackaging of other CDO tranches.

S&P has cut its ratings on several hundred billions of dollars of structured-finance securities in the past year as expected losses continue to mount along with mortgage defaults.

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