Interdiffusion in a nondegenerate, exhaustion range, binary ionic semiconductor is investigated using the recently improved theory for the concentrations of defects in crystals and Wagner's phenomenological flow equation. The composition dependence of the interdiffusion constant is found to be determined by the ratio of the ion jump frequencies, the Schottky constant, and the intrinsic concentration of conduction band electrons. The removal of the restriction to an exhaustion range semiconductor and the replacement of the basic assumption of local electroneutrality are discussed.