The business was previously handled by Media Planning Group.
PHD won the account after a pitch process handled through the AAR. Online media buying will remain with MPG, through its Media Contacts division.
The appointment of PHD is effective from January when it will work with Euro RSCG Wnek Gosper, which was appointed earlier this year and introduced the "what's your perfect trip?" advertising.
Expedia has attempted to position itself as a website that offers more than just flights and is trying to get consumers to use it as more of a comprehensive holiday website.
CHI created a large marketing campaign for a January sales promotion, involving price reductions and holidays, flights, car hire and accommodation.
This involved TV, radio, press, outdoor and online activity.
Expedia launched in October 1998. Its main shareholder, Microsoft, sold its stake last year to the owner of Ticketmaster, USA Interactive.
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