The charge acceptance of single layer organic photoreceptors consisting of α-type Cu-phthalocyanine pigments and hydrazone charge transport materials dispersed in thermo-setting acrylic polymer is found to decrease when the surface is exposed to ambient NO_X gas. The electronic surface properties of the photoreceptors are investigated by treating the surface with HNO₃ aqueous solutions and by measuring the applied voltage-current characteristics. Internal photo-emission experiments determine the energy-level relationship between metal electrodes and the photoreceptors. The hole injection from the treated surface can be attributed to the adsorption of acceptor ions at the surface. The improvement of charge acceptance based on this conclusion is suggested.