Novel catalyst compositions for copolymerizing ethene and carbon monoxide comprise: a)a compound of a Group VIII metal chosen from palladium, cobalt and nickel, b)an anion of an acid with a pKa of less than 6, and c)a bidentate ligand of the general formula R<1>R<2>-M-R-M-R<3>R<4>, wherein M represents phosphorus, arsenic or antimony, R is a bivalent organic bridging group containing two or three carbon atoms in the bridge and R<1>, R<2>, R<3> and R<4> represent hydrocarbon groups which may or may not be substituted with polar groups, on the understanding that at least one hydrogen atom in at least one of the groups R<1>, R<2>, R<3> and R<4> is substituted with a polar substituent. |