Unexpectedly severe losses on pools of commercial real estate that were sold to investors by the Resolution Trust Corp. have prompted Standard & Poor's Corp. to review its credit ratings on the RTC's commercial-mortgage-backed securities.

During the summer, the agency twice downgraded one RTC issue backed by about $325 million of mortgages secured by Southern California apartment complexes, lowering its ratings to BBB from AA. Last week, it placed another RTC issue backed by just over $300 million of "multifamily" loans, also on Southern California property, on credit watch with negative implications.

Gale Scott, director of surveillance for the rating agency, said on Monday that all of S&P's ratings on RTC securities are "under review."

From 1991 through the early part of September, the agency has pooled and sold $4.5 billion of loans secured by apartment complexes, $9.1 billion of loans secured by commercial real estate, and $2.4 billion on nonperforming real estate loans.

To receive investment grade ratings, the RTC created seemingly impregnable reserve funds to cover any losses on the loans.

But for many of the RTC securities rated by S&P, underlying loans are not just going into default at a higher rate than expected. "We've found that once the servicer does foreclose, the losses are significantly higher than originally anticipated," Ms. Scott said. The reserve fund for the 1991 M2 pool that already was downgraded has been reduced to $77.4 million from $122 million, S&P said. The reserve fund for the 1991 M4 series that was placed on credit watch has been depleted to $87.6 million from $107.4 million.

Ms. Scott said the situation is of particular concern to the investors who bought the lowest-rated tranches of the securities in search of high yields. They would sustain any loss not covered by the reserve fund. A spokeswoman for the RTC would only say that the agency is "aware" of the S&P actions.

She noted that the agency is not obligated by law to replenish reserve funds, but she added that the RTC would probably review that option if the situation arose.

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