The electrochemical behaviour of thick films of platinum hydrous oxides is unusual in the extent to which kinetic, rather than thermodynamic, factors influence both the formation and reduction of such deposits on the parent metal. These two processes occur at widely different potentials, the separation in some instances being ca. 2 V. On the basis largely of selective oxide reduction techniques, which are described here for the first time in the case of hydrous oxide deposits, 13 experimentally distinguishable PtPt oxide electrode systems may be produced; these are of course accessible only over limited ranges of potential. The presence of four distinguishable types of oxide means that the study of anodic oxide films and anodic processes, e.g. oxygen gas evolution, on platinum is more complex than is generally realized. Unusual cyclic voltammetric responses were observed with platinum in acid following reduction of an aged thick hydrous oxide deposit. Cathodic responses were observed at low potentials, even on the positive sweep, which were attributed to the presence of a recalcitrant hydrous oxide residue, i.e. one that was reduced only slowly even at 0.0 V (RHE) and, clearly for kinetic reasons, showed very little tendency to reduce above ca. 0.2 V (RHE).