The dissolution kinetics of polymer powders as isolated grains or in beds were studied theoretically and experimentally. A simple model developed for the dissolution of a dispersion of isolated grains with an arbitrary size distribution was confirmed experimentally using polysaccharides in water. Based on experimental data for the dissolution of lump-forming powder beds, modeled using a bimodal population of grains and lumps, a robust technique was developed for analyzing experimental data, which is independent of the details of lump formation. A qualitative picture of lump formation is described, based on the idea that during the initial hydration of a powder bed a "pore blocking" condition quickly develops due to the formation of a gel layer around the dry grains. Once formed, lumps dissolved the same way and with the same kinetic description, as individual, dispersed grains.