Last month Microsoft made an unsolicited offer to purchase Yahoo. The bid was initially rejected, but executives from the two companies met last week to discuss the proposed deal, according to the Wall Street Journal.
"We would be concerned by any kind of acquisition of Yahoo by Microsoft," chief executive Eric Schmidt told Reuters.
"We are concerned that there are things Microsoft could do that would be bad for the internet. We would hope that anything they did would be consistent with the openness of the internet, but I doubt it would be," he added.
Schdmidt's comments follow a ruling by a European Court in 2004 that Microsoft abused the near-monopoly power of its Windows operating system to gain advantage over its competitors.