Process for the preparation of copolymers

A process for the preparation of copolymers of carbon monoxide with an olefinically unsaturated compound comprising copolymerizing the monomers in the presence of a catalyst composition based upon (a) a salt of a Group VIII metal, and (b) a cation which is selected from cationic oxidants and cations of the general formulae RnH4-nN+, RmH3-mC+ and RmH3-mO+, wherein n is 0, 1, 2, 3 or 4, m is 2 or 3, and R represents a hydrocarbyl group and/or two or three of R together with the atom carrying a positive charge represent a ring structure, on the understanding that when n is 3, at most two of R together with nitrogen represent a heterocyclic ring structure, and when n is 4, three of R together with nitrogen represent an aromatic heterocyclic ring structure.