a catalyst system for the selective carbonylation of conjugated dienes in the presence of a hydroxyl - group - containing compound in the liquid phase comprising a combination of : palladium or a compound of ; an organic phosphorus , antimony or arsenic derivative according to the formula : ## str1 ## wherein r 1 represents an aryl group substituted with at least one electron - attracting group and r 2 and r 3 each represent individually an aryl group or an aryl group substituted with at least one electron - attracting group and r 2 and r 3 each represent individually an aryl group or an aryl group substituted with at least one electron - attracting group and wherein q represents phosphine , antimony or arsenic ; and an acid with a pka from about 2 . 5 to about 4 . 0 , said acid being a condensed aromatic carboxylic acid or a benzoic acid substituted with at least one electron attracting group selected from the group consisting of halogen , hydroxy , and methoxy .