Relaxation of monodisperse and polydisperse electrorheological (ER) suspensions under linear deformation is investigated. The electrostatic polarization model is employed to describe ER suspensions, and solutions to the equations of motion are obtained by dynamic simulation and regular perturbation methods. Sources of relaxation events are identified by examining partial rheological functions that are defined in the text. Relaxation of both monodisperse and polydisperse suspensions is found to be confined to approximately the same limited range of relaxation times, although polydisperse suspensions may produce relaxation events over a very broad range of relaxation times. Events occurring at large relaxation times arise from relatively weak forces that do not significantly influence the extent of relaxation. Other relaxation mechanisms and potential sources of dispersion broadening are also discussed.