Google announced late on Friday that it would purchase online advertising firm DoubleClick for $3.1 billion in cash from its two primary owners, the private equity firms of Hellman & Friedman and JMI Equity.

In addition to picking up a second form of advertising to compliment its AdSense technology, Google gets demographic data and the tools to gather such information, plus DoubleClick’s clients.

The combination of Google , which was also in talks to acquire DoubleClick. The Wall Street Journal reported of the talk in late March, saying DoubleClick was talking to the Redmond company “and other potential suitors.” Other suitors were believed to be Yahoo and AOL.

If the deal is approved, it should close by the end of the year. The DoubleClick acquisition comes just six months after Google paid $1.65 billion for YouTube.

DoubleClick was an early pioneer in online advertising and one of the few dot com success stories to survive the dot com implosion earlier in the decade. The company was taken private in 2005 for by the two private equity firms for $1.1 billion.

Last year, DoubleClick acquired Klipmart, the US’s largest provider of online video advertising and management solutions for Web publishers.

Google declined to give financial details on DoubleClick. According to the New York Times, DoubleClick generated about $300 million in revenue last year.

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