Ruthenium dioxide-coated titanium electrodes when used as cathodes in water electrolysis cells are metastable systems which display high catalytic activity and unexpected stability. At the potentials involved the oxide films clearly should undergo reduction; the barrier to the latter process seems to arise due to hydroxylation of the outer layers of the thermally prepared oxide; the reduction of hydrous oxide films in general is inhibited by the intervention (as the primary reduction product) of a high energy state of the metal. The catalysis of the hydrogen gas evolution reaction at the oxide/solution interface is attributed to the involvement of a metastable cyclic redox mediator system involving multivalent Ru(OH)(x) surface species.