CME Group ( CME) and Nymex Holdings ( NMX) on Monday confirmed they are in preliminary discussions about a possible merger. Chicago-based CME Group, which operates the Chicago Mercantile Exchange, is offering Nymex shareholders $36 in cash and 0.1323 shares of its common stock, in exchange for each Nymex share. Based on Friday's closing prices, the potential deal values Nymex at $119.22 a share, a premium of 11.3%. The two exchanges said in a joint statement they are entering a 30-day exclusive negotiating period. CME Group said it expects to maintain trading floors in New York. Nymex said it would repurchase 816 New York Mercantile Exchange memberships upon closing for up to $500 million. The potential deal comes less than two weeks after NYSE Euronext ( NYX), which operates the New York Stock Exchange, said it was acquiring its smaller rival, the American Stock Exchange, for $260 million in stock. Nymex shares were rising 8.2% to $116.02 in early trading. CME Group's stock was up 2.5% to $644.86.