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][OctSO₄]. 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 H₂ 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 (ΔG^‡). A greater loss in entropy for the oxidative addition of H₂ over alkene insertion causes the ΔG^‡ of the oxidative addition to be greater than the ΔG^‡ of alkene insertion at high temperature.