A range of platinum deposits, equivalent thicknesses (δ) 0.2–2.5 nm, have been synthesised on carbon and reduced titania (TiO_x) supports using physical vapour deposition on (10 × 10) arrays of electrodes. For δ < 1.0 nm, discrete platinum centres are formed and the TiO_x supported platinum show two distinct characteristics: (a) a strong positive shift in the potential for the oxidation of monolayers of CO with decreasing loading of Pt leading to an inability to oxidise the CO on the lowest loadings and (b) a strong negative shift in the potential for the reduction of oxygen. Both observations can be understood in terms of an increase in the irreversibility of the Pt/PtO couple at such surfaces. The same trends, although significantly weaker, are seen with the carbon supported platinum, δ < 1.0 nm, and it is suggested that the Pt/PtO couple on carbon shows intermediate kinetics between Pt on TiO_x and bulk Pt. These results have significant implications for understanding the mechanism of oxygen reduction on supported Pt catalysts and hence for the search for alternative supports to platinum for ORR electrocatalysts.