AOL — which Verizon Communications acquired this year for $4.4 billion — is offering $1.75 per share of Millennial Media common stock, a 33% premium over Millennial's closing stock price Wednesday.

The deal for Millennial Media bolsters AOL's mobile-ad capabilities, as it fights rivals like Facebook and Google for share of marketing spending as consumers spend more time on smartphones and tablets.

"AOL is well positioned as consumers spend more and more time on mobile devices, and as advertisers, agencies and publishers become more reliant on programmatic monetization tools," AOL president Bob Lord said in announcing the pact.

Millennial Media, founded in 2006, has about 575 employees. The company is based in Baltimore with additional U.S. offices in Atlanta, Boston, New York and San Francisco, and international offices in Hamburg, London, Paris, Singapore and Tokyo. Millennial Media's portfolio of assets includes acquisitions of TapMetrics, Condaptive, Metaresolver, Jumptap and Nexage.

AOL said the deal, subject to customary regulatory approvals and other closing conditions, is expected to close this fall.

In 2013, AOL acquired programmatic video-ad platform provider Adap.TV for $405 million.