Any deal between Yahoo and Microsoft could be "bad for the internet", according to the head of Google.
Chief executive Eric Schmidt said that Google was "concerned" about a deal and said it could have implications for the "openness" of the internet.
Last month Microsoft proposed a buyout of Yahoo for $44.6bn, a deal that was rejected by Yahoo's board.
At the time, experts said that the buyout was an attempt by Microsoft to challenge Google's dominance.
"We would be concerned by any kind of acquisition of Yahoo by Microsoft," Mr Schmidt said at a conference in Beijing.
"We would hope that anything they did would be consistent with the openness of the internet, but I doubt it would be."