The agency saw off competition from Wieden & Kennedy, Campbell Doyle Dye and the incumbent, Euro RSCG Wnek Gosper, to win the business. The review did not affect any of Expedia's other incumbent agencies.
Over the coming months, Expedia will be looking for CHI to help it promote its full travel packages and encourage customers to use the website for more than just booking flights.
The appointment comes as Expedia prepares to launch a multimillion-pound marketing campaign for what it claims is its biggest January sales promotion, involving price reductions and holidays, flights, car hire and accommodation.
The campaign, which will incorporate TV, radio, press, outdoor and online work, is built around Expedia's Flight Plus service. Flight Plus is an online booking engine that enables customers to build their own package holidays and short breaks.
Expedia's marketing director, Beverly Shaw, who joined from Coca-Cola in November, said: "CHI's combination of strategic thinking and creative approach is well-suited to driving our brand forward to the next stage of its development."
One of CHI's managing partners, Johnny Hornby, added: "We are thrilled to be working with such a fast-growing and dynamic company, with a brand and product that excels in its category. Expedia.co.uk provides a really exciting strategic and creative challenge."
Expedia launched in October 1998. Its main shareholder, Microsoft, sold its stake last year to the owner of Ticketmaster, USA Interactive.