An experimental approach that directly measures light-induced electrostatic surface potentials at the gas-liquid interface using the combined techniques of the atomic force microscope and the scanning surfacepotential microscope is presented. Photosystem I (PSI) reaction centers, one of the molecular photovoltaic structures of green plants, convert light energy into electrical energy. We report the orientation and measurement of light-induced photovoltages at the air-water interface by PSI solutions entrained in the pores of microchannel glass. The data indicate that illuminated PSI reaction centers reversed the sign of the water surface electrostatic potential from negative to positive and that additional illumination further increased the positive value of the potential. The sign of the light-induced photopotentials indicates that the reaction centers are oriented with their electron-transport vectors pointing toward the water phase. Electrostatic potentials as a function of PSI concentration are presented for concentrations of active PSI up to 1 muM. Orienting biomolecules at the airwater interface might be an enabling technology for new classes of sensors.