Microsoft announced the biggest acquisition in its history by buying digital marketing firm aQuantive for $6 billion in an all-cash deal.

"The advertising industry is evolving and growing at an incredible pace, moving increasingly toward online and IP-served platforms, which dramatically increases the importance of software for this industry," Microsoft CEO Steve Ballmer said in a statement.

The aQuantive acquisition enables Microsoft to strengthen relationships with advertisers, agencies and publishers by enhancing the company's advertising platforms and services beyond its current capabilities to serve MSN.

"It is a big bet on advertising monetisation for the long-term growth of the company and this is a significant step forward. The online advertising market would be worth $40 billion this year and is growing at 20% a year," said Kevin Johnson, president of Microsoft's platform and services division.

The deal is expected to be completed in the first half of Microsoft's fiscal year 2008.

PTI