NEW YORK — Microsoft Corp., not wanting to get left behind rivals in an online advertising boom, agreed yesterday to pay $6 billion in cash to acquire aQuantive Inc., a leading agency for Internet ads which also has powerful technology that serves display and banner ads to other Web sites.

The eye-popping premium of 85 percent Microsoft is paying for aQuantive reflects a heated race for the few remaining online advertising businesses, as media and technology companies jockey for position in the Internet advertising market.

The deal caps a month of furious activity in the sector which began in mid-April with Google Inc.'s $3.1 billion purchase of DoubleClick Inc., a company that provides technology used by some Web site publishers to deliver ads to viewers.

Google is already the leading provider of keyword search advertising, which offers "sponsored" links along with search results. Companies such as DoubleClick and aQuantive help deliver "display" ads on Web sites such as banners and boxes, which lead users to the advertisers' Web sites.

For Microsoft, getting aQuantive could jump-start its online advertising business, which lags far behind Google and Yahoo Inc. because of the lower traffic on its own destination Web site, MSN. The deal "certainly keeps them in the competitive arena with Google," A.G. Edwards analyst Denise Garcia says.

The aQuantive technology will allow Microsoft to deliver ads to third-party Web sites, something that should mesh with Microsoft's existing plans for delivering ads onto other platforms, such as video games on its Xbox, which connects to the Internet. Microsoft currently delivers ads mostly to its own Web sites, such as MSN.

aQuantive also has a significant interactive advertising agency called Avenue A/Razorfish, which buys, sells and creates online advertising — much like a traditional advertising agency, but online.

Microsoft, whose MSN site has struggled behind search leader Google and also Yahoo, is hoping the acquisition will help sell and deliver advertising on the Internet, a business that is on pace to grow nearly 30 percent this year, way ahead of other forms of advertising.

Kevin Johnson, the head of Microsoft's platforms and services division, which includes the Windows operating system and online services such as MSN, said there was a "competitive process" to acquire aQuantive, but he declined to talk about other bidders.

Piper Jaffray analyst Aaron Kessler estimates the online advertising market to be worth about $25 billion this year. Over the next five years, Kessler said, he expects annual growth of about 17 percent.

Johnson said Microsoft was "committed to earning a bigger slice of that market opportunity" with the acquisition, but some analysts expressed surprise at the richness of the price.

However, with rival Google now in control of DoubleClick — a major server of online ads — many experts believe it was imperative that Microsoft get in the market if it wants to have any kind of scale in online advertising.

The deal came just a day after advertising conglomerate WPP Group PLC acquired another online ad company, 24/7 Real Media Inc., for $649 million.

Yahoo made its move last month, agreeing to buy the portion of privately-held online ad exchange Right Media Inc. that it didn't already own for $680 million. In January, advertising conglomerate Publicis Groupe purchased the online advertising company Digitas.