The performance of rolls and belts used in charging, development and transfer processes of electrophotography depends critically on dielectric relaxation of the semi-insulating overcoat layers. In these materials, the time rate of dielectric relaxation is shown to be determined by charge injection from the bias and charge mobility, rather than by the bulk resistance. Thus, characterization of these devices, especially in high-speed printing applications, where time allotted for each process becomes limited, should emphasize the transient characteristics and spatial variations of dielectric relaxation.