Process that serves to remove all ionized substances from a solution. Most common is the exchange process where cations and anions are removed independently of each other: first, positively charged ions are removed by a cation exchange resin in exchange for a chemically equivalent amount of hydrogen ions; next, negatively charged ions are removed by an anion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions introduced in this process unite to form water molecules. The term is often used interchangeably with demineralization. The cation resin is regenerated with an acid and the anion resin is regenerated with sodium hydroxide (caustic soda).