The European Commission has fined four firms 345m euros ($444m; £235m) for fixing prices and operating a cartel in the sale of acrylic glass products.
Commissioner Kroes says cartels are "a scourge"
The punished companies were French group Arkema, British firms ICI and Lucite, and Irish business Quinn Barlo.
They were found to have fixed prices and exchanged sensitive information between 1997 and 2002.
With trade names including Plexiglas and Perspex, acrylic glass is used in areas such as lighting and technology.
The fines are the fourth-largest handed out by the European Union for price fixing.
"These fines will serve as a cold shower for the management and shareholders of all these companies, who have to realise that cartels cannot and will not be tolerated," said EU Competition Commissioner Neelie Kroes.
German company Degussa was also involved in the cartel, but escaped a fine because it co-operated with competition authorities.
Arkema, which was recently spun-off from French oil giant Total, was fined 219m euros, ICI 91.4m, Lucite 25m and Quinn Barlo 9m euros.
The companies will have the chance to appeal against the decision at the European Court of First Instance.
Earlier this month, the Commission hit seven chemicals companies with total fines of 388m euros for operating a similar price fixing cartel.
Degussa was also involved in that case, but was again let off for co-operating with the EU.