PURPOSE: To obtain a catalyst composition suitable for the preparation of a polymer of carbon monoxide and an ethylenically unsaturated compound by incorporating a Group VIII metal such as palladium and a specific bisphosphine.

CONSTITUTION: A desired catalyst composition comprises (A) a Group VIII metal and (B) a bisphosphine having the formula [wherein R2 is an aryl group having an alkoxyl substitutent in the ortho position to the phosphorus atom (the aryl group is connected to the phosphorus atom); R3 is an divalent bridging group containing 3 carbon atoms in the bridge]. Further, the desired composition incorporated with component (B) in an amount of 0.5-2 moles per 1 g-atom group VIII metal (preferably, palladium) is preferred.