In a process for the preparation of dimethyl butanedicarboxylates, wherein(a) butadiene or a hydrocarbon mixture containing butadiene is reacted with carbon monoxide and methanol in the presence of a tertiary nitrogen base and a cobalt carbonyl catalyst at from 80.degree. to 150.degree. C. under superatmospheric pressure,(b) the greater part of the tertiary nitrogen base and any excess hydrocarbons are removed and(c) the methyl pentenoate thus obtained is reacted with carbon monoxide and methanol, in the presence of a cobalt carbonyl catalyst and the residual amount of tertiary nitrogen base, at from 140.degree. to 200.degree. C. under superatmospheric pressure, to give the dimethyl butanedicarboxylate, the improvement that the reaction mixture obtained in stage (a) after reaction of the butadiene is cooled until it separates into two phases, and a part-stream of from 20 to 80% by volume of the lower phase is recycled to stage (a).