AOL took another step Wednesday in its transformation into an online advertising firm with the acquisition of venture-backed Quigo in a deal that sources said was worth at least $340 million.

The main financial backers of 7-year-old Quigo are Highland Capital, Institutional Venture Partners, and Steamboat Ventures, the venture capital arm of  Walt Disney. Overall, New York-based Quigo has raised about $30 million in venture capital.

The acquisition marks another bet by AOL on its “Platform A” strategy launched in August 2006. That plan calls for cobbling together an integrated online advertising company, Platform A, from Quigo and a series of prior acquisitions, including Advertising.com, a $435 million acquisition in 2004, Tacoda, acquired in July for a reported $275 million as well as video ad company Lightningcast, cell phone ad company Third Screen Media, and German advertising management firm Adtech, whose purchase prices were not disclosed.

The stakes are high for AOL, once reigning Internet king as America’s largest provider of dial-up Internet service. In third-quarter results released Wednesday, AOL’s parent, Time Warner said the unit’s revenue fell 38 percent to $1.2 billion. Subscription revenue fell 56 percent, but was partly offset by a 13 percent increase in advertising revenue. As of September 30, the AOL Internet service had 10.1 million U.S. subscribers, a decrease of 851,000 from the previous quarter and 5.1 million from the comparable 2006 quarter.

There has been widespread speculation that Time Warner would spin off AOL, seen as a drain on its parent, if the subsidiary’s financials stabilize. For the nine months ended September 30, AOL accounted for 12 percent of the revenue of Time Warner, whose broadcast, movie, and publishing empire includes HBO, CNN, People, Sports Illustrated, Time, “ER,” and the Harry Potter movies.

Henry Vogel, Quigo’s chief revenue officer, said that Quigo, whose AdSonar technology buy ads on web sites based on specific pages, sections, topics, or keywords, attracted attention from potential suitors in recent months. Though the New York City company did not conduct an auction, it did hire investment banker Goldman Sachs to provide advice.

“There were a lot of people interested,” he said. “It’s a good time to be in our space. We think it’s the second or third inning in an advertising revolution.”

In recent months, advertising firm DoubleClick fetched $3.1 billion from Google, while aQuantitative brought in a price of $6 billion from Microsoft.