The exchange of ions of the same charge between a solution (usually aqueous) and a solid in contact with it. The process occurs widely in nature, especially in the absorption and retention of water-soluble fertilizers by soil. For example, if a potassium salt is dissolved in water and applied to soil, potassium ions are absorbed by the soil and sodium and calcium ions are released from it.

The soil, in this case, is acting as an ion exchanger. Synthetic ion-exchange resins consist of various copolymers having a cross-linked three-dimensional structure to which ionic groups have been attached. An anionic resin has negative ions built into its structure and therefore exchanges positive ions. A cationic resin has positive ions built in and exchanges negative ions. Ion-exchange resins, which are used in sugar refining to remove salts, are synthetic organic polymers containing side groups that can be ionized. In anion exchange, the side groups are ionized basic groups, such as −NH₃⁺ to which anions X⁻ are attached. The exchange reaction is one in which different anions in the solution displace the X⁻ from the solid. Similarly, cation exchange occurs with resins that have ionized acidic side groups such as −COO⁻ or −SO₂O⁻, with positive ions M⁺ attached.

Ion exchange also occurs with inorganic polymers such as zeolites, in which positive ions are held at sites in the silicate lattice. These are used for water-softening, in which Ca²⁺ ions in solution displace Na⁺ ions in the zeolite. The zeolite can be regenerated with sodium chloride solution. Ion-exchange membranes are used as separators in electrolytic cells to remove salts from sea water and in producing deionized water. Ion-exchange resins are also used as the stationary phase in ion-exchange chromatography.