The kinetic study of the hydroformylation of styrene catalysed by the rhodium/1,3,5-triphenyl-1H-phosphole (TPP) system has been facilitated by the fact that a catalytic system having two TPP ligands per Rh atom is maintained all along the catalytic cycle and no dissociation of a TPP ligand has to be considered during this cycle. This has allowed us to propose a model of mechanism with an association complex between the styrene and the unsaturated HRh(CO)(TPP)(2) species. An analytical equation of the reaction rate has been established which acceptably characterises the behaviour of the reaction rate according to the concentration of the various species. This study reveals that the selectivity between linear and branched alkyl-rhodium is under thermodynamic control and the reversibility of the transformation of the alkyl into acyl-rhodium isomers has not clearly been established but suggested by the observations. An inhibiting effect of the produced aldehydes, through complexation with rhodium has also been put in evidence. This study emphasizes also the complex role of the CO and H2 partial pressures on the rate of reaction. A single catalytic cycle, only differentiated by the formation of linear or branched aldehydes, based on these observations and consistent with the kinetic equation is proposed. (C) 2004 Elsevier B.V. All rights reserved.