The influence of an additional gas on the hydroformylation of cyclohexene in the presence of Co-2(CO)(6)(PBu3)(2) has been tested. The rate of the hydroformylation is reduced by the presence of an appropriate amount of dinitrogen, argon or xenon as additional gas. The conversion decreases as the pressure of the additional gas increases. Helium, on the other hand, does nor show any influence. These results are in agreement with the previous data reported for the hydroformylation of the same olefin in the presence of Co-2(CO)(8) even if the entity of the reduction of the reaction rate is now less evident. The reduced influence of the additional gas may be attributed to the more severe conditions necessary to perform the reaction or to the higher stability of the catalytic system. The analogy between these two catalytic systems is confirmed by the comparable influence displaced by the additional gas. This influence on the reaction rate may be attributed, on a molecular basis, to a competition among the additional gas and dihydrogen and/or olefin to a coordinatively unsaturated place on the catalytically active complex. The formation of an additional gas containing complex reduces the concentration of the active cobalt intermediate available for the catalysis and, as a consequence, the hydroformylation rate. These data are not. sufficient to identify the step of the catalytic process influenced by the presence of the additional gas, however, indicate the involvement of a dihydrogen or an olefin containing complex in the rate determining step of the hydroformylation. The formation of a cobalt complex containing an additional gas as ligand (dinitrogen, argon or xenon) in the conditions required to perform the hydroformylation is supported by these experiments.