A photoelectrochemical response was found for a methyl viologen pendant polysiloxane film coated on an indium tin oxide (ITO) electrode dipped in an aqueous electrolyte solution containing O2. Both the current direction (cathodic or anodic) and the extent of the photoresponse depended on the substitutional ratio of methyl viologen on the polymer chain. This behavior was not explained by simple photoinduced electron transfer from the excited cation radical of the methyl viologen to O2. The mechanism is discussed for steady state 02 and cation radical concentrations and with respect to O2 permeability in the polysiloxane film.