An investigation of the kinetics of propene hydroformylation in the gas phase has been conducted over a silica-supported Rh-sulfoxantphos complex stabilized by the ionic liquid [bmim][OctSO(4)]. The reaction temperature was found to have a strong effect on the kinetics of n- and iso-butanal formation. For both products, it was observed that increasing the temperature decreased the apparent activation energy, altered the reaction orders with respect to reactants, and decreased the molar ratio of n- to iso-butanal. The observed changes in the kinetics are discussed in terms of the generally accepted mechanism for olefin hydroformylation and are attributed to a change in the rate-determining step (RDS). It is concluded that at low temperature, the RDS is alkene insertion into an Rh-H bond but becomes the oxidative addition of H-2 at high temperature. The change in the RDS is rationalized.in terms of a change in the elementary step with the largest Gibbs free energy of activation (Delta G double dagger). A greater loss in entropy for the oxidative addition of H-2 over alkene insertion causes the Delta G double dagger of the oxidative addition to be greater than the Delta G double dagger of alkene insertion at high temperature. (C) 2012 Elsevier Inc. All rights reserved.