The structure of the electrostatic latent image formed by exposure of a corona-charged photoreceptor must be maintained prior to development. To meet this requirement, useful photoreceptors have high dark resistivities. If the photoreceptor surface is conductive, the electrostatic latent image will degrade with time. The time-dependent changes in the structure of an electrostatic latent image are analyzed, using a model with surface resistivity as the only adjustable parameter. The analysis offers a method for determining the surface resistivities of thin polymer films. Surface resistivities are reported for photoreceptors that have been modified by various surface treatments.