By means of digital acquisition and processing of potential–decay transients, following interruption of anodic currents for O₂ evolution at oxidized Ni surfaces, the behaviour of the adsorbed intermediate species in the heterogeneous reaction is quantitatively evaluated for various initial currents and for several temperatures. The method is of interest in electrocatalysis and electrode kinetics since it provides, by its nature, information on the adsorbed species that are kinetically involved as intermediates in the steady state of the reaction proceeding at appreciable net rates, rather than on some strongly bound species that may be adsorbed but not participate as the active intermediate in the reaction sequence. The behaviour of the O₂ evolution reaction at oxidized Ni anodes is characterized by two regions of potential-dependent pseudocapacitance that reflect the potential dependence of the surface density of the intermediate species: one probably involving the surface region of the nickel oxide film and another involving a potential-dependent state of oxidation of bulk-phase-type oxide. The behaviour of the adsorbed intermediates, OH or O, is intimately connected with the oxidation states of Ni as Ni³⁺ and Ni⁴⁺ in the surface region of the oxide film at which O₂ is anodically evolved. Experiments at various temperatures reveal an important dependence of the adsorption behaviour of the intermediates on that variable; the effect arises for kinetic reasons.