A range of reduced titania (TiO_x) supported platinum electrocatalysts have been synthesised using physical vapour deposition on arrays of electrodes. Surfaces with equivalent thicknesses of platinum in the range 0.2–2.5 nm on a uniform layer of TiO_x have been synthesised on 10 × 10 arrays. The arrays have been used to study the surface redox chemistry of the supported platinum as well as the oxidation of a monolayer of carbon monoxide on the platinum. It is shown that below an equivalent thickness of 0.8 nm, there is a positive shift in the potential for the oxidation of the platinum surface and a negative shift for the reduction of the oxide with decrease in the platinum loading. These shifts show that it is the kinetics of the platinum/platinum oxide couple that change with platinum loading; the couple becomes increasingly irreversible with decreasing loading. The peak potential for the oxidation of the monolayer of carbon monoxide also shifts positive and broadens with decreasing platinum loading; these trends are again particularly marked below an equivalent thickness of 0.8 nm while below 0.4 nm no CO oxidation peak is observed although it could be confirmed that CO is adsorbed on such surfaces. Again, these changes with platinum loading are associated with the irreversibility of the platinum/platinum oxide couple. At low equivalent thicknesses, it is impossible to form the oxidised platinum species within the carbon monoxide monolayer essential to the commencement of oxidation of the CO monolayer.