An improved catalyst composition for the production of linear alternating polymers of carbon monoxide and at least one ethylenically unsaturated hydrocarbon is formed from a compound of palladium, the anion of a strong non-hydrohalogenic acid and a bidentate ligand of phosphorus wherein each monovalent phosphorus substituent is phenyl substituted by at least one alkoxy group on a ring carbon atom ortho to the atom through which the group is connected to a phosphorus atom, and the divalent alkylene bridging group between the phosphorus atoms has a single substituent (other than hydrogen) having atoms of carbon, hydrogen and optionally oxygen.