New York Mercantile Exchange (Nymex) executives are mulling over an unsolicited offer for a majority stake in the exchange from private equity firm Parthenon Capital.

Boston-based Parthenon has expressed interest in taking a 60% stake in the exchange. Nymex issued a statement that a bid had been made of up to $2m per seat, but had no further comment regarding the timeline for the offer. The exchange held its monthly board meeting on Wednesday.

Ernest Jacquet, co-ceo of Parthenon, told...