Developing a solid state Photosystem I (PSI) modified electrode is attractive for photoelectrochemical applications because of the quantum yield of PSI, which approaches unity in the visible spectrum. Electrodes are constructed using a Nafion film to encapsulate PSI as well as the hole-scavenging reox me PY diator Os(b) Cl The photoactive electrodes generate photo, currents of 4 mu A/cm2.+1,vhen illuminated with 1.4 mW/cm(2) of 676 nm band-pass' filtered light. Methyl viologen (MV4) is present in the electrolyte to scavenge photoelectrons from PSI in the Nafion film and transport charges to the counter electrode. Because MV2+ is positively charged in both reduced and oxidized states, it is able to diffuse through the cation permeable channels of Nafion. Photocurrent is produced when the working electrode is set to voltages negative of the 0s3+/Os2+ redox potential. Charge transfer through the Nafion film and photohole,scavenging at the PSI luminal surface by Os(bpy)(2) Cl-2 depends on the reduction of Os redox centers to Os2+ via hole scavenging from PSI. The optimal film densities of Nafion (10 pg/cm 2 Nafion) and PSI (100 trg/cm2 PSI), are determined to provide the highest photocurrents. These optimal film densities force films to be thin to allow the majority of PSI to have productive electrical contact with the backing electrode.