The European Commission has fined seven companies a total of 388m euros ($491m; £267m) for operating a cartel in the chemicals sector.
Commissioner Kroes said the fines were deliberately high
The firms are Belgian firm Solvay, France's Arkema, Dutch business Akzo Nobel, Finland's Kemira, Spain's FMC Foret, and Snia and Edison from Italy.
Following a Commission probe, they were found to have exchanged commercially important and confidential information.
In addition, they limited output and allocated market share and customers.
They were also found to have fixed target prices of two products, hydrogen peroxide and perborate.
The Commission said two other companies were involved in the 1994 to 2000 cartel but not fined: Germany's Degussa and France's Air Liquide.
Degussa, a repeat offender, was let off a fine of 130m euros for providing the Commission with information about the cartel.
Air Liquide could not be fined since it had left the market in 1998, and the Commission's right to impose financial penalties had expired in the intervening time.
"Cartels are unacceptable corporate behaviour that deprive customers of the benefits of the single market," said EU Competition Commissioner Neelie Kroes.
"These high fines take into account that certain companies are repeat offenders.
"Directors and shareholders alike should ask why these practices were allowed to continue."
Hydrogen peroxide is an oxidising agent used in the pulp and paper industries for bleaching textiles.
It is also used as a disinfectant in sewage treatment.
Perborate is used mainly as an active substance in synthetic detergents and washing powders.
In 2000 the combined European market value for both products totalled 470m euros.
Spanish-based FMC Foret is owned by US parent group FMC.