Electrochemical performances of DSA-type oxide electrodes (eg, RuO₂, IrO₂, Co₃O₄) suggest that proton exchange between the oxide surface and the solution accompanies the surface modification. Experimental data point also to the existence of “inner” surface regions from which proton exchanges is retarded. Electrochemical data have been supplemented with tritium exchange experiments, and in depth proton profile determinations by means of nuclear reaction analysis. A consitstent picture has been obtained enabling conclusive evidence to be achieved for the mechanism of surface hydration of oxides, its involvement in electrochemical redox transition, and for the nature of the different behaviour of RuO₂ and IrO₂ with respect to Co₃O₄.