The raids on March 25 targeted makers of peroxides, solvents and specialized acids, the European Union's head office said in a statement.
The European Commission did not name the companies or countries involved, but Belgium's Solvay, TotalFinaElf SA subsidiary Atofina of France, and German chemical giant BASF AG all said they were involved.
"Solvay is fully committed to co-operating actively with the competition authorities," the company said in a statement.
The Commission said EU and national antitrust inspectors looked for evidence that "exchange of confidential commercial information in relation to sales" allowed companies to rig prices, limit output and divide up territories.
It said the investigation is "at a preliminary stage" and that the companies involved may have done nothing wrong.
Atofina spokesman Gilles Galinier told Dow Jones Newswires inspectors "photocopied papers and stayed here a good part of the day." He added the company is "fully cooperating."
"We provided the Commission with relevant documents" BASF spokeswoman Jennifer Moore-Braun told Dow Jones Newswires.
Under EU rules companies found guilty of fixing prices can be fined up to 10 percent of worldwide annual sales.
The highest accumulated fine ever levied in Europe for running a cartel -- 855 million euros ($918 million) -- was imposed on a group of vitamin producers in 2001.