The equilibrium isotope effect (EIE) for oxidative addition of H-2 and D-2 to Ir(PMe2Ph)(2)(CO)Cl has been measured over a large temperature range, thereby demonstrating that the inverse (<1) BE previously observed at ambient temperature becomes normal (>1) at high temperature (>90 degreesC). The temperature dependence of the BE for oxidative addition of H-2 and D-2 to Ir(PH3)(2)(CO)Cl has been calculated using the geometry and vibrational frequencies obtained from DFT (B3LYP) calculations on Ir(PH3)(2)(CO)ClH2 and Ir(PH3)(2)(CO)ClD2, and is in accord with the experimentally observed transition from an inverse to normal BE for oxidative addition of H-2 and D-2 to Ir(PMe2Ph)(2)(CO)Cl: the EIE is calculated to be inverse between 0 and 510 K, reach a maximum value of 1.15 at 867 K and then slowly decrease to unity as the temperature approaches infinity. This deviation from simple van't Hoff behavior, and the occurrence of a maximum in the EIE, is the result of the entropy term being temperature dependent. At low temperature, the enthalpy term dominates and the BE is inverse, whereas at high temperatures the entropy term dominates and the BE is normal. The observation of both normal and inverse EIEs for the same system indicate that inferences pertaining to the magnitude of an isotope effect at a single temperature may require more detailed consideration than previously realized.