The nature of oxide-supported metal catalysts may change in oxidizing conditions: Studies on the correlation between the Rh phase in the structure of the Rh/Al2O3 catalyst and the catalytic performance for the reduction of NO by H2 to N2 (on reduced, metallic sites) and N2O (on oxidized sites) reveal that the phases of the supported metal species can be interconverted on time scales that can be deterministic in terms of the activity and selectivity of the catalysts.