Water deionization consists of removing electrically charged disolved (ionized) substances. Non-electrically charged substances, such as organic compounds, are not affected while passing through the resin. The deionization process is performed by an ionic exchange resin. These can be of cationic exchange, if they substitute dissolved cations in the water (sodium, calcium, magnesium, iron and others), or anionic exchange, if substituing dissolved anions in the water (carbonate, chloride, nitrate, etc.) by hydroxile radicals. To remove all water ions a combination of resins can be used, one anionic and another cationic, both connected in a series.

Ionic exchange resins have a limited exchange capacity. When exhausted, they cannot keep exchanging ions, and must be subjected to a regeneration process to reach their full capacity again.