The surface photovoltage of ZnO resin layer has been used as the basis for an imaging process generally known as chargeless electrophotography. This paper explores effects of the ambient air pressure and the layer temperature on the surface photovoltage of ZnO resin layers. Experiments were made by using ZnO-silicone resin layer, ZnO-alkyd resin layer, and ZnO-acryl resin layer. The surface potential of the dark adapted layers were measured while the ambient air pressure decreased, and the surface photovoltage and its decay curve were measured under various ambient air pressures. ZnO-silicone resin layer showed a remarkably high sensitivity in terms of surface photovoltage to the ambient air pressure changes. Marked variations were observed in the surface potential of the dark adapted layers both in air and in a vacuum of 5 x 10(-5) Torr when the layer temperature had been slowly raised. The surface potential exhibited a maximum peak when silicone resin was used as a binder and a minimum peak when alkyd resin or acryl resin was used, both peaks being registered at a temperature slightly higher than room temperature in air. After the layers had been annealed for a few hours at a high temperature, relationships between the surface photovoltage and the layer temperature were measured while the layer temperature decreased. The surface photovoltage and its decay of the ZnO-silicone resin layer revealed higher sensitivity to the changes of ambient air pressure and layer temperature than to those of other ZnO-resin layers. This difference is accounted for by a specific property of the silicone resin that enforces adsorption of the water molecule onto the surface of ZnO. Some applications of the above experiments are also discussed.