A buyout would provide Microsoft with access to DoubleClick's Dart technology, which monitors how internet adverts perform, and boost its ability to fight Google for online advertising market share.
DoubleClick is currently wholly owned by private-equity company Hellman & Friedman LLC, which recently had the unit valued as part of a possible stock market listing.
However, the advertising company could be sold outright to a bidder, with Microsoft and a number of other parties currently in the running, according to the Wall Street Journal.
A sale at $2bn would make Hellman & Friedman LLC a profit of $0.9bn since it bought DoubleClick in 2005.
DoubleClick's sales last year were quoted by unnamed sources in the paper to be just $150m.
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