[en] Graphical abstract: - Highlights: • Ru and Ru thermal oxides films were prepared on Ti substrate. • Material properties of OER catalytic films were monitored throughout the OER cycles. • OER activity decreased by different degradation rates for each Ru oxide film. • Anhydrous RuO_2 inhibited dissolution of Ru metal and the formation of hydrous RuO_2. - Abstract: The activity and stability of Ru metal and its thermal oxide films for the oxygen evolution reaction (OER) were investigated. The metallic Ru films were prepared by electrodeposition on a Ti substrate and then thermally oxidized at various temperatures under atmospheric conditions. During long-term operation of the OER with cyclic voltammetry (CV) in H_2SO_4 electrolyte, changes in the properties of the Ru and its thermal oxides were monitored in terms of their morphology, crystal structure, and electronic structure. In the initial stages of the OER, all of the Ru thermal oxide films underwent an activation process that was related to the continuous removal of low-activity Ru oxides from the surface. With further cycling, the OER activity decreased. The rate of decrease was different for each Ru film and was related to the annealing temperatures. Monitoring of material properties indicates that the amount of stable anhydrous RuO_2 is important for OER stability because it prevents both the severe dissolution of metallic Ru beneath the oxide surface and the formation of a less active hydrous RuO_2 at the surface.