Google is apparently allowing third-party advertisers to track consumers using the company's ad network, a practice that raised concerns from one privacy advocate.

Some of the third-party ad servers and ad agencies that Google has approved to deliver ads through its network engage in behavioral advertising practices that require tracking consumers, said Jeffrey Chester, a privacy advocate and frequent critic of Google's privacy practices.

"Google is the most important digital media company," Chester, the executive director of the Center for Digital Democracy (CDD), wrote in an e-mail. "Its activities should be as transparent as possible to consumers (let alone regulators)."

Google announced this month that it has begun to allow third parties to deliver ads on the Google content network. Several of the third parties listed by Google in its announcement engage in behavioral advertising, Chester said.

For example, Mediaplex, which offers ad-serving products and services, says it offers a "behavioral targeting platform." The company's products allow "advertisers to target users based on previous actions and deliver the most relevant messages," according to Mediaplex marketing materials.

Mediaplex was one of 12 companies that Google said it has certified to deliver ads through Google's ad network.

"Google has now sanctioned behavioral targeting on its network, and users have no idea what the implications are," Chester said.

Google, in its announcement on third-party ads and in communications to Chester, has said it is using a certification process to screen third-party advertisers.

The company has developed "comprehensive certification policies," according to information from Google passed on by Chester. The certification policies outline how cookies can be used on the Google network, and require that third-party ad servers cannot use cookies in connection with personal information unless the user opts in, according to Google. Consumers can also opt out of any tracking mechanism, Google told Chester.

Google spokespeople didn't immediately respond to a request for comments.

Google's explanation isn't clear on how the behavioral advertising will work, Chester said. While Google has talked about consumer opting in to behavioral advertising, it appears the third-party ads will automatically appear on the Google network, he said.

"The companies are using the Google network, and you don't even know about it," Chester added. "Google's explanation that its rules prohibit cookies from being connected with [personal information] doesn't really address the key consumer privacy issue. Behavioral-targeting cookies are used to track and target individual users."

Google officials have said that the company doesn't engage in behavioral advertising. Company officials have questioned the effectiveness of the practice.

Chester and other privacy advocates have raised concerns about online consumer tracking in recent months. Citing privacy concerns, the CDD and two other privacy groups asked the U.S. Federal Trade Commission to block Google's US$3.1 billion acquisition of ad server DoubleClick, announced in April 2007. The FTC, however, voted in December to approve the deal.