The effects of variations in carrier mobility and its field dependence on the xerographic discharge characteristics of a photoreceptor have been explored theoretically. Particular emphasis has been placed on the dependence of optimum exposure to reach optimum contrast potential and the latter values on mobility. The discharge characteristics are divided into three categories: (i) flash exposure where the exposure time is much shorter than the carrier transit time, (ii) emission‐limited discharge which is realized by higher mobility photoreceptors, and (iii) space‐charged‐perturbed or space‐charge‐limited discharge which is seen in low‐mobility photoreceptors. It is shown that in the latter case the optimum contrast potential is dependent on the time of observation (synonomous with development time).