Tributylphosphine n-propyl- (Ia) and iso-propyl (Ib) cobalt tricarbonyl have been prepared from NaCo(CO)(3)(PBu3) and the proper alkyl halide. The behaviour of Ia and Tb under hydroformylation conditions (T = 120-180 degrees C, p(CO) = 5 atm, p(H2) = 50 atm in the presence or absence of an excess of an olefin or PBu3) has been tested. Ia and Tb gave the expected isomeric alcohols with a selectivity higher than 95%. The role of these complexes in the hydroformylation reaction has been discussed. (C) 1997 Elsevier Science S.A.