Yahoo has announced the acquisition of online advertising network BlueLithium for $300 million in cash.

Blue Lithium specializes in selling performance-based display ads across a network of Web sites. Like search-related text advertising, performance banners let advertisers pay for exposure to customers only when people respond to the ad, whether it’s by agreeing to receive more information or by taking some action. The privately held company, which also sells behavioral targeted advertisiments, is ranked the fifth largest ad network in the United States and second largest in the United Kingdom with 145 million unique visitors per month.

The acquisition of BlueLithium enables Yahoo! to accelerate its advertising, product, and engineering roadmaps, and gives the company increased capabilities to sell and measure performance-based campaigns both on and off the Yahoo! network.

BlueLithium and Yahoo! share a common goal of providing both advertisers and publishers with high quality inventory and the essential targeting and analytical tools that are necessary to reach the right consumers at the right times. The addition of BlueLithium is the logical next step in creating the largest and most effective online ad network globally, which also includes inventory on Yahoo!’s owned and operated properties, the Yahoo! Publisher Network, and the Right Media Exchange. We believe that together Yahoo! and BlueLithium will help drive the next evolution of the online advertising industry. [BlueLithium]

The deal comes five months after Google bought online ad giant DoubleClick for $3.1 billion and less than four after Microsoft acquired aQuantive for $6.1 billion.