Net Ratings is paying just a fraction of the $414m (£290m) Media Metrix paid for Jupiter Communications when it bought it in June 2000. However, the price tag represents a 200% premium on the firm.
Dave Toth, NetRatings chief executive, said, "By acquiring Jupiter Media Metrix and eRatings, we are taking two giant strides toward realising the vision we have had since the beginning to build a premier internet research and measurement company."
VNU acquired NetRatings when it bought market research giant ACNielsen earlier this year for £1.5bn.
The Jupiter deal is expected to be completed in the first quarter of next year.