The metal hexacyanometalate films, Prussian blue (PB), Ni hexacyanoferrate (NiHCF), and Cu hexacyanoferrate (CuHCF), were studied for their ability to poise the potential of an oxide-covered metal in a stable, passive region of voltage. This requires the films to catalyze oxygen reduction, and oxygen reduction on PB films was found to be as large as 0.14 mA/cm2. However, no net oxygen reduction was observed on NiHCF and CuHCF films. When PB films were galvanically coupled to Ti electrodes, the open-circuit potential was stabilized at 0.38-0.52 V vs. SCE even at area ratios (Ti/PB) as high as 10. PB films directly deposited onto Ti electrodes were also studied. The films could only be reduced at a negative potential (-0.18 V vs. SCE) near the flatband potential for TiO2, and electrochemical oxidation did not occur under dark, deaerated conditions. Upon exposure of reduced PB on TiO2 to dissolved O2, or to illumination, the PB film was oxidized and the coupled system adopted a stable potential between +0.2 and 0.3 V.