Ion exchange is a chemical reaction in which ions in a solution are replaced by others of like charge. An insoluble solid is used that has an open, net-like molecular structure: a zeolite, or a synthetic organic polymer called an ion-exchange resin, whose composition and properties can be tailored for the use required. The solid has attached anionic groups (negatively charged), which are neutralized by small mobile cations (positively-charged ions) in the interstices. It is these cations which are exchanged for others when a solution is passed through. The principle of anion exchange is similar.

Ion exchange is used for softening hard water, purifying sugar, and concentrating ores of uranium and the noble metals. Ion-exchange chromatography is used to separate the rare earths, and in chemical analysis.