The homogeneous hydroformylation of 1-octene using either of the heterobimetallic complexes [(eta(5)-C5H5)Ru(mu-CO)(2) {mu-eta(2)-Ph2P(CH2)PPh2}RhCl2] (1) and [(eta(5)-C5H5)Ru(mu-CO)(2){mu-eta(2)-HC(PPh2)(3)}RhCl2] (2) as catalysts yields both n-nonanal and 2-methyloctanal. The best results were obtained with complex 2, having the tripod {HC(PPh2)(3)} bridging ligand, which showed the highest regioselectivity for linear aldehydes. Although the rate of reaction is slower for these heterobimetallic catalysts than for related monometallic species, the selectivity for terminal aldehydes is relatively greater. Two mechanisms involving bimetallic cooperativity between the Ru and the Rh metal centres, either in the form of an intramolecular hydride transfer, or in the form of the Ru centre acting as a labile ligand, are proposed. (C) 2003 Elsevier Science B.V. All rights reserved.