By hiring more revenue officers, the I.R.S. could collect more than $9 billion each year and spend only $296 million  or about three cents on the dollar  to do so, Charles O. Rossotti, the computer systems entrepreneur who was commissioner from 1997 to 2002, told Congress four years ago.

I.R.S. officials on Friday characterized those figures as correct, but said that the plan Mr. Rossotti had proposed had been forestalled by Congress, which declined to authorize it to hire more revenue officers.

Critics of the privatization plan point not only to the higher cost but also to what they say is a greater potential for abuse. With private companies in the mix, they say, debtors could more easily be tricked into paying money to scam artists using spoof Web sites or other schemes, a problem the I.R.S. alerted taxpayers to in April. Brady R. Bennett, collections director for the I.R.S., said that by 2008, about 350,000 past-due tax records will be distributed among about 10 private debt-collection agencies. To guard against fraud, he said, the agencies will contact taxpayers only by telephone or mail  not the Internet  and will instruct them to send all payments directly to the United States Treasury, not the private collection agency.

One of the three companies selected by the I.R.S. is a law firm in Austin, Tex., where a former partner, Juan Peña, admitted in 2002 that he paid bribes to win a collection contract from the city of San Antonio. He went to jail for the crime.

Last month the same law firm, Linebarger Goggan Blair & Sampson, was again in the news. One of its competitors, Municipal Services Bureau, also of Austin, sued Brownsville, Tex., charging that the city improperly gave the Linebarger firm a collections contract that it suggested was influenced by campaign contributions to two city commissioners.