Google has announced the purchase of Postini, paying $625 million for the global on-demand communications security and compliance solutions firm.

Postini's services include message security, archiving, encryption and policy enforcement that can be used to protect a company's email, instant messaging and other web-based communications.

"With this transaction, we're reinforcing our commitment to delivering compelling hosted applications to businesses of all sizes," said Google CEO Eric Schmidt. "With the addition of Postini, our apps are not just simple and appealing to users, they can also streamline the complex information security mandates within these organizations."

Google has been on a buying streak of late, inking two deals in July alone. Last week, Google announced that it had purchased GrandCentral Communications, a firm that allows users to manage all mobile and land phone lines from a single account.