The deal values Nymex at $1.35 billion, and is seen as a stepping-stone to an initial public offering.

The proceeds would go to Nymex’s 816 seatholders. The Nymex board will recommend the offer to its shareholders and members at a September 29 meeting.

“General Atlantic has a proven record of building value in the exchange sector,” said Nymex chairman Mitchell Steinhause. “We believe this transaction provides Nymex with the right balance of continued autonomy, open-outcry trading protection