A kinetic study of the photocatalytic oxidation of water at a n-RuS2 semiconducting single crystal has been undertaken on the basis of photocurrent transients (photocurrent-time behavior as a function of the polarization potential, illumination intensity, and temperature) and electrolyte electroreflectance experiments. The main factor defining the catalytic activity of RuS2 for water oxidation, both in the dark and under illumination, is a low overpotential (eta approximate to 0.3 V), which is comparable to that of the RuO2 catalyst for oxygen evolution at darkness. Evidence has been given that eta is determined by the E-o(Ru-s-OHo/Ru-s-H2O) redox potential, which strongly depends on the bonding energy of Ru surface species with OHo radicals generated by direct oxidation of adsorbed water molecules (interfacial Ru-peroxo-type complex formation). This bonding energy increases as the RuS2 surface becomes oxidized under anodic polarization and reaches its maximum value at the potential of the S2RuO2/RuS2 transition (VIII Ru oxidation state), Further oxidation of the Ru-peroxo-type complexes leads to oxygen evolution at a rate which increases with the degree of oxidation of the Ru surface active centers. Although O-2 evolution probably already takes place on Ru(VI) surface sites, high evolution rates (current densities) are only reached under oxidation state VIII. However, in this state (idealized S2Ru(VIII)O-2) Ru-S surface bonds are weakened and occasionally broken, contributing to RuS2 dissolution with generation of volatile RuO4 and SO42--soluble ions as the main corrosion products. This phenomenon may be attributed. to the reaction in acidic medium of H2O molecules with Ru(VIII) surface species, giving rise to the formation of unstable intermediate complexes.