European Commission competition officials are looking into the licensing strategies behind the two rival next-generation DVD formats.
Both HD DVD and Blu-ray promise better picture and sound quality
The two rival systems are HD DVD, which has been developed by Toshiba, and Blu-ray, which has been created by a consortium led by Sony.
Each promises better picture and sound quality than existing DVDs can offer.
A Commission spokesman said it was asking the firms for more information. Sony said it was co-operating.
"We have sent a letter earlier this month to the makers of HD DVD and Blu-ray to request information about licensing," said the Brussels spokesman.
The Commission wants to know details of how the makers of the two new formats plan to license out their products to hardware manufacturers wanting to make new DVD players, as well as to firms wishing to produce the new discs themselves.
Once the Commission's anti-monopoly officials receive more information, they can decide whether or not to start an official inquiry.
Sony said in a statement: "There are no indications of any complaint, nor of any antitrust concerns on the part of the Commission or anyone else."
Both HD DVD and Blu-ray have already been released in the US - HD DVD in April and Blu-ray in June.
Sony's consortium behind Blu-ray also includes Philips, Samsung and Sharp.
Toshiba was not immediately available for comment.