Ion-exchange resin, any of a wide variety of organic compounds synthetically polymerized and containing positively or negatively charged sites that can attract an ion of opposite charge from a surrounding solution. Ion-exchange resins are light and porous solids, usually prepared in the form of granules, beads, or sheets. When immersed in solution, the resins absorb the solution and swell; the degree of swelling is dependent on the polymeric structure and the total ion concentration of the solution.