Privacy advocates have objected to the deal, saying it would give the two firms unprecedented access to information about consumers. The Commission has said privacy considerations are outside the scope of its authority over mergers.
The deal would combine Internet search engine giant Google's dominance in pay-per-click Web advertising with DoubleClick's market-leading position in flashier display ads.
The planned acquisition won approval from the Federal Trade Commission in December.
Microsoft, Google's competitor in internet advertising industry, bought aQuantive for $6 billion while Yahoo acquired BlueLithium for $300 million and AOL bought Tacoda for an undisclosed amount.