Yahoo is the nearest challenger to Google, the most popular search engine in Europe. Bing, launched last year, remains a tiny contender with a European market share Microsoft estimates at two percent.
In December, the pair finalised the terms of deal, which was first announced in July last year. However before they can press ahead, they need the approval of the EU.
All mergers and joint ventures involving large corporations are routinely given a one-month examination by the Commission's competition department.
If any concerns about the deal's impact on fair competition arise then officials can open a second phase review of an additional four months.
Microsoft and Yahoo hope that the transaction will close early this year. The pair also said they welcome the broad support the deal has gotten from key players in the advertising industry.