A spokesman said the raids concerned 'alleged anti-competitive practices' - its term for illegal price-fixing or market-sharing cartels - in the PVC market.
The Commission spokesman declined to give the names of the firms but confirmed that the dawn visits took place last week. However, industry sources said one of the targets was European Vinyls Corporation, a joint venture between ICI and Enichem of Italy.
John Catt, public relations manager of EVC, said his firm, the leading producer of PVC in Europe, had been raided in Britain and Brussels. He denied that EVC had been involved in any collaborative attempt with other PVC producers to control prices or markets.
The sources said the other companies raided included BASF, Royal Dutch/Shell, Hoechst, the British arm of Norsk Hydro, and Solvay. They said that the officials had the help of a team from the Office of Fair Trading.
The Commission has the power to enforce fines worth up to 10 per cent of firms' annual turnover for involvement in illegal cartels.
After similar raids in the 1980s, the commission imposed fines of 23.5 million ecu ( pounds 16.8m) in 1988 on a large group of PVC producers for operating an illegal cartel. However, the fines were later quashed by an EC court on procedural grounds.