Mixed oxides of nominal composition Ir0.3Ti(0.7-x)PtxO2 (0 less than or equal to x less than or equal to 0.7) were prepared by thermal decomposition at 400 degrees C of suitable precursors on Ti supports. The surface properties were investigated ex situ by SEM and EDX, and in situ by cyclic voltammetry in 1 mol dm(-3) HClO4 solutions. The morphology changes at about 30 mol% Pt; at lower Pt contents the surface is enriched with Ir; at higher contents it is enriched with Pt. Cyclic voltammetry showed that the surface concentration of active sites decreases with increasing Pt content O-2 evolution was studied by means of current-potential curves and reaction order determinations. The mechanism was found to change going from Ir to Pt surface sites. The electrocatalytic properties were separated from surface area effects by normalizing the current to unit surface charge. The specific activity turned out to decrease by a factor of 10 as Ti is replaced by Pt, which in turn displaces Ir from the surface of the active mixed oxides.