Abstract: A sustained release drug delivery composition and a method of making same are provided which comprises microspheres containing a pharmaceutically active agent, a core of a ion-exchange resin and a polymeric coating completely surrounding the core wherein the coating is water-insoluble and hydrolytically stable in physiological environments. The coating is non-erodible in physiological environments and is characterized by a diffusion constant suitable for the pharmaceutical agent to diffuse from the core to an aqueous environment at a predetermined release rate.