Expedia Inc. and the Priceline Group have been gobbling up one rival after another over the past two and a half years, to the point where few existing online booking sites in the United States are in a position to challenge them — except perhaps one-time Expedia sister brand TripAdvisor.

Expedia’s acquisition today of one-time rival Orbitz┬áis the latest in a series of volleys that has seen the two giants trade purchases of metasearch sites, car rental companies, and traditional booking sites.

In their battle for online supremacy, Expedia and Priceline have opened their wallets and spent collectively more than $7.6 billion over the last two years. Expedia has spent more than $3.2 billion on acquisitions since 2013, while Priceline has spent over $4.4 billion.

Related: Analysis: It’s Expedia Against Everyone Else After Orbitz and Travelocity Deals

With the exception of Orbitz, companies are listed by the year in which the acquisition closed, not when it was announced.

Expedia Acquisitions

Company/YearPriceBackground
2015
Orbitz$1.6 billionread story
Travelocity$280 millionread story
2014
Wotif$658 millionread story
Auto Escape$85 millionread story
2013
Trivago$632 million (62% stake)read story

Priceline Acquisitions

CompanyPriceBackground
2014
OpenTable$2.6 billionread story
Buuteequndisclosedread story
Hotel Ninjasundisclosedread story
Qlika$3 millionread story
2013
Kayak$1.8 billionread story

To a lesser extent, TripAdvisor has been an active acquirer since its split from Expedia in December 2011. The company spent $400 million on acquisitions in 2014.

TripAdvisor’s largest purchase was tours and activities brand Viator in August of 2014 for $192 million, but it has bought numerous smaller companies, often for very small amounts, as it builds both its media, hotel technology and restaurant reservations services. These amounts are usually small, but they add up: 2014’s tally included $208 million for six additional companies, including Vacation Home Rentals, Tripod, Lafourchette, MyTable, Restopolis and Iens.

TripAdvisor states that the seven acquisitions it made in 2014 accounted for about $37.4 million or 3 per cent of total revenue.

Acquisitions in 2013 cost TripAdvisor $34.8 million net of cash acquired.

TripAdvisor Acquisitions

Company/YearPriceBackground
2014
Viator$192 millionread story
VacationHomeRentalsundisclosedread story
Tripbodundisclosedread story
Lafourchetteundisclosedread story
My Tableundisclosedread story
Restopolisundisclosedread story
Iens$11 millionread story
2013
TinyPostundisclosedread story
Jetsetterundisclosedread story
CruiseWiseundisclosedread story
Niumbaundisclosedread story
GateGuruundisclosedread story
Oysterundisclosedread story