The activity of rhodium for oxygen gas evolution in base is significantly enhanced by the presence of a thick, hydrous oxide film which is produced by repetitive potential cycling. An increase in oxygen evolution rate of up to three orders of magnitude was obtained on growing these films; the latter were found to be stable to dissolution (in short term tests) at potentials below ≃1.70V (RHE). A decrease in Tafel slope of 75 to 48 mV decade−1 was observed on going from a hydrous oxide-free to a hydrous oxide-covered rhodium surface at potentials less than ≃1.55V (RHE), while a slope of ≃120 mV decade−1 was observed in the linear Tafel region above 1.55V (RHE) in all cases. A reaction order with respect to hydroxide ion activity was also determined. A mechanism for oxygen evolution was proposed involving cyclical formation-decomposition of higher rhodium oxide species. A feature of the kinetic treatment was the incorporation in the rate equation of a term that took into account the variation of the Rh(IV) concentration at the interface, at constant overpotential, with change in solution pH. © 1990, The Electrochemical Society, Inc. All rights reserved.