The probe follows complaints from three companies including Microsoft-owned shopping comparison site Ciao!
The commission will look into whether the US giant penalises potential competitors in its search rankings, and whether it uses its dominance in the European search market to keep advertising prices at a high.
A blog posted by Google's senior competition counsel Julia Holtz said the US search giant started receiving complaints from Ciao! about its standard terms and conditions after Microsoft acquired the business in 2008.
Microsoft initially took its case about Google's practice of setting minimum bid levels in its advertising auction model to the German competition authority, but this has now been transferred to Brussels.
Holtz also said that the other two complaints were from UK price comparison site Foundem and a French legal search engine, ejustice.fr.
Both businesses have complained that Google's algorithms leave their results in the search rankings low because they are a vertical search engine, like Moneysupermaket.com and Opodo, and so, a direct competitor to Google.
Hotlz said: "This kind of scrutiny goes with the territory when you are a large company. We understand how important rankings can be to websites, especially commercial ones, because a higher ranking typically drives higher volumes of traffic. We are also the first to admit that our search is not perfect, but it's a very hard computer science problem to crack.
"Though each case raises slightly different issues, the question they ultimately pose is whether Google is doing anything to choke off competition or hurt our users and partners.
"This is not the case. We always try to listen carefully if someone has a real concern and we work hard to put our users' interests first and to compete fair and square in the market."
.jpg)
.jpg)