5to1, based in Los Angeles, describes itself as an "online advertising alliance" made up of about 20 major publishers who offer brand advertisers premium ad space online "at mass scale," according to its website.

Yahoo foresees leveraging 5to1's technology and roster of publisher partners and advertiser clients to boost its sales of premium ad inventory.

The deal is expected to close in this year's second quarter. The 5to1 staff will join Yahoo's Ad Marketplaces group.

Yahoo has traditionally been a strong player in the market for premium online ads, which, compared with other online ad formats, are more expensive, graphically rich and prominently placed.

Customers for these ads are typically large companies with strong brands that run online campaigns to market their products and services, like movies, soft drinks and cars, to consumers.