An additional gas such as dinitrogen, argon or xenon, present in the reaction medium in high concentration, affects the hydroformylation of cyclohexene, hex-1-ene or styrene in the presence of RhHCO(PPh3)(3). The rate of hydroformylation of cyclohexene decreases as the amount of the additional gas increases: this effect may be attributed to a competition between the additional gas and one of the reagents, alkene, hydrogen or carbon monoxide for a coordinative unsaturated site available on the catalytically active intermediates. An analogous behaviour is shown when a terminal olefin (hex-1-ene or styrene) is hydroformylated in the presence of a low pressure of xenon. On the contrary, a very high pressure of dinitrogen or argon is required to show a decrease of the initial rate of the hydroformylation of the same terminal olefins. Finally, there is no or a slightly positive influence of a high pressure of helium or methane on the initial rate of the hydroformylation of the above reported olefins. This behaviour may be explained assuming an interaction between the additional gas and the rhodium catalyst. (C) 2003 Published by Elsevier Science B.V.