Deionizers utilize a cation resin regenerated with acid as well as a anion resin regenerated with caustic to remove all of the ions, thus removing most of the TDS (Total Dissolved Solids) in the water.

In the context of water purification, ion-exchange is a rapid and reversible process in which impurity ions present in the water are replaced by ions released by an ion-exchange resin. The impurity ions are taken up by the resin, which must be periodically regenerated to restore it to the original ionic form. (An ion is an atom or group of atoms with an electric charge. Positively-charged ions are called cations and are usually metals; negatively-charged ions are called anions and are usually non-metals).