Preparation of catalyst solution

A process for the preparation of copolymers of carbon monoxide and one or more ethylenically unsaturated compounds by reacting carbon monoxide and one or more ethylenically unsaturated compounds under polymerization conditions in the presence of a diluent in which the copolymers are insoluble or virtually insoluble and in the presence of a catalyst solution prepared by (a) combining a palladium compound, a bisphosphine of the general formula R.sup.1 R.sup.2 P--R--PR.sup.3 R.sup.4 (I) wherein each of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 independently represents an aromatic hydrocarbyl group which is substituted with an alkoxy group and R represents a bivalent bridging group containing at least two carbon atoms in the bridge, and a lower alcohol as solvent, and

• (b) admixing the obtained combination after a generation period G, during which a temperature T in the range of 0.degree.-70.degree. C. is maintained, with an acid having a pKa of less than 2, whereby the said generation period G is selected such that the value for G (in hours) and the temperature T (in .degree.C.) are related as ##EQU9##