The firms were involved in a "rip-off", Neelie Kroes said.
The European Union has fined six makers of power transformers a total of 67.6m euros (£62.6m; $99.6m) for running a cartel that artificially hiked prices.
The three European and three Japanese firms operated like "a gentleman's club" by agreeing not to sell in each other's markets, the EU said.
ABB of Switzerland, Alstom and Areva T&D of France, and Fuji Electrics, Hitachi and Toshiba were all fined.
Siemens of Germany took part in the cartel but escaped a fine.
It had revealed the scheme's existence to the EU executive, and so did not get a financial penalty, EU competition chief Neelie Kroes said.
The probe found that executives of the seven companies met each year to reaffirm their cartel deal, and in talks used code names for their companies to avoid detection.
"Customers and tax-payers all over Europe suffered from this cartel for a number of years," Ms Kroes said.
"The commission has now put an end to this rip-off by the self-appointed 'gentlemen'."
Power transformers modify the voltage in electricity transmission networks.