Experimental studies on amorphous selenium under potential probe configuration have shown a well defined residual potential. By analytically relating this quantity to material parameters, we have been able to separate out the bulk and surface trapping from each other and also from complicating photogeneration effects. Apart from obtaining the deep trapping lifetime, we were able to identify a surface region in amorphous selenium which had different trapping properties from the bulk. The knowledge about this surface region is essential in a complete description of the photogeneration process. We believe that the study of residual potential can lead to valuable information about high resistivity materials. © 1972.