As pointed out in previous work, hydrous oxide deposits grown on polycrystalline platinum by the potential cycling technique frequently consist of two components, designated HO1 and HO2. The HO1 material appears to be an uncharged Pt(IV) species, possibly Pt(OH)4 or PtO2 . nH2O. It reduced on the negative sweep, in both acid and base, at ca. 0.4 V/RHE. The second component (HO2) reduced in a negative sweep in acid at ca. 0.2 V/RHE, but exhibited a significant (though not total) stability, obviously of kinetic rather than thermodynamic origin, in base at room temperature at potentials as low as -0.5 V/RHE. Since, from a thermodynamic viewpoint, Pt(IV) oxide species should reduce at ca. 1.05 V/RHE, both HO1 and HO2 exhibit anomalous stability. The greater stability of HO2 in base as compared with acid at 25-degrees-C is attributed to the enhanced OH- ion stabilization of the hydroxy complex involved in solutions of high pH. At slightly elevated temperatures (> 50-degrees-C) this trend was reversed : thick HO2 films reduced at more positive potentials in base than in acid. The absence of foreign, e.g. HSO4-, anions (which are generally present in HO2 deposits) in base is assumed to favour conversion of HO2 to the more readily reduced HO1 during the course of reaction at elevated temperature in solutions of high pH. On a typical negative sweep (0.8-0 V/RHE), the extent of HO2 film reduction in base at 25-degrees-C increased with decreasing sweep rate. The presence of a quasi-stable HO2 deposit on platinum in base had an inhibiting effect on electrolytic processes such as hydrogen gas evolution and ferricyanide reduction-however, the initially HO2-coated electrode retained a significant electrocatalytic activity for the oxidation of hydrazine, despite the presence of quite a significant covering of unreduced oxide throughout the experiment.