Kayak began integrating TripAdvisor's 60 million hotel reviews and ratings on Kayak.com and Kayak UK, and plans to introduce them globally.
Kayak also offers user hotel reviews from Kayak visitors as well as from sources such as Travelocity, IgoUgo, Citysearch, MyTravelGuide and Epinions.
Robert Birge, Kayak's chief marketing officer, says reviews from sources other than TripAdvisor will be given less weight.
"We're maintaining [these other reviews], but TripAdvisor reviews will have priority based on quantity, currency, management of the content and global brand recognition of TripAdvisor as the leader in reviews and opinions," Birge says.
You'll notice in the above image that TripAdvisor ratings and the number of reviews for a property are emphasized atop the display for each property.
As part of Kayak's hotel recommendations filter, users will be able to exclude hotels from their search which have not been reviewed by TripAdvisor -- making TripAdvisor ratings and review all the more important for hotels offering rooms through Kayak and online travel agencies.
Users previously could filter hotel reviews based on professional reviews from Frommer's and Budget Travel and these options will remain in place, as well.
Kayak has been trying to resolve the user review issue in a number of ways over the years.
Kayak acquired hotel review site Travelpost and then sold it in March 2010.
The number of Travelpost reviews, coupled with reviews Kayak collected on its own and gathering from third-party providers could match the scale of TripAdvisor's millions of reviews.
Now, TripAdvisor reviews will dominate Kayak's hotel reviews.
This is all a far cry from just three years ago when Kayak CEO Hafner said TripAdvisor was "polluting the web" as a result of its over-commercialisation.
The TripAdvisor reviews on Kayak's US and UK sites were slated to be rolled out today.