A kinetic study of the homogeneous hydroformylation of 1-hexene to the corresponding aldehydes (heptanal and 2-methyl-hexanal) was carried out by using a rhodium catalyst formed by the addition of 1 equiv. of 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) to the complex Rh(acac)(CO)(2) under mild reaction conditions (80 degrees C, 2-10atm of syn-gas) in toluene; linear to branched ratios (l/b) varied from 1.3 to 5.8, depending on the reaction conditions. The reaction rate is first order with respect to the concentration of Rh, fractional order with respect to 1-hexene concentration and zero order with respect to dissolved hydrogen concentration. Increasing the CO pressure up to a threshold value of 2.1 arm accelerates the reaction, further increments inhibit the reaction. Complex RhH(CO)(kappa(3)-triphos) was isolated and characterized by IR and NMR (H-1 and P-31(H}). The kinetic data and related co-ordination chemistry are consistent with a mechanism involving RhH(CO)(kappa(2)-triphos) as the active species and the migratory insertion of the alkene into the metal-hydride bond as the rate limiting step. This catalytic cycle is rather similar to that proposed for RhH(CO)(PPh3)(3) and for RhH(CO)(2)(dppe): however, the presence of a triphos ligand co-ordinated in a kappa(2) mode through the cycle resulted in l/b ratio higher than those obtained in systems containing bidentated phosphines. (c) 2008 Published by Elsevier B.V.