The equilibrium electrostatic potential of interstellar grains, subjected to an ultraviolet radiation field in a dilute ambient plasma, is calculated for a variety of radiation fields and plasma parameters, and for two different grain materials. For an environment typical of an H I region, the results show that the various components of interstellar grains in the multicomponent model may charge to opposite potentials, thus favoring the formation of mixed composite grains due to Coulomb attraction. In H II regions, a three-shell model of the grain charge is presented, which includes a sphere of positively charged grains around the central star bounded by a transition region in which grains of low and of high photoyield are oppositely charged. Here grain growth and the formation of mixed composite grains may take place. In the outer shell all grains are charged negative. Estimated radii of these shells are given for exciting B0 and O5-type stars. (auth)