(EN) A catalyst composition which is based upon (a) a source of nickel cations and (b) a bidentate ligand of the general formula (I): R1R2M1-R-M2R3R4 wherein M1 and M2 represent independently phosphorus, nitrogen, arsenic or antimony, R1, R2, R3 and R4 independently represent optionally substituted aliphatic or alicyclic groups and R represents a bivalent bridging group of which the bridge which extends directly between the atoms M1 and M2 consists of at most two bridge atoms; and a process for the preparation of copolymers of carbon monoxide and an olefinically unsaturated compound comprising contacting the monomers in the presence of the said catalyst composition.