Abstract The hydroformylation of cyclic dienes in benzene using Rh(CO 2Me)(CO)(PPh 3) 2 as catalyst was studied. The formation of mono- or di-aldehydes was critically dependent on ring size, the smaller cyclic dienes favouring dihydroformylation. Intermediate alkene-aldehydes could be isolated under appropriate conditions, indicating that initial attack on the diene was by hydroformylation. Under hydroformylation conditions, these intermediates underwent either hydroformylation or hydrogenation, depending on the ring size. Linear dienes gave complex mixtures of products.