Under the terms of the deal, comScore Arbitron Online Radio Ratings will measure listening for TargetSpot's national network of 600 online radio stations, which are owned by various partners, including CBS Radio (now partnered with AOL), as well as Entercom Communications Corp.

The move is part of a strategy to make online radio more appealing to media buyers, which also includes the introduction of frequency caps and flighting, custom networks and compatibility with other online metrics. For smaller advertisers, TargetSpot has an online self-service feature that allows them to make audio ads online.

ComScore Arbitron will provide TargetSpot partner stations with average quarter-hour ratings and cumulative audience estimates, broken down by dayparts and demographics--the same currency used for traditional radio sales.

The TargetSpot announcement arrives along with a deluge of new initiatives from Internet radio broadcasters and ad networks. Earlier this week, Clear Channel Radio's online division announced partnerships with Gracenote for lyrics and Clearspring for widgets, as well a deal with StudioNow putting its network of freelance video ad producers at the disposal of local advertisers.

CBS Radio also owns a small stake in TargetSpot.