Demineralization or deionization is the removal by ion exchange of all ions in solution, both cations and anions. That requires use of both cation exchange resin in H+ form and anion exchange resin in OH form. That means that the cation resin is regenerated with acid (H+ ion), and the anion resin is regenerated with strong base or OH ion. The cation resin exchanges all cations for H+ ion, and the anion resin exchanges all anions for OH ion. The two combine to form water with nothing left over, so that the treated water is similar to distilled water quality. It is not identical to distilled water, because the ion exchange process does not remove electrically neutral molecules such as sugars, alcohols, many pesticides, VOCs, and other un-ionized organic compounds which may be present.

The two resin types may be combined in a “mixed bed” or in separate beds or cartridges. It is difficult (but possible) to regenerate a mixed bed, because the two resins must be separated first. This is done by “fluidizing the bed” (backwashing, or operating up-flow) so that the resin types separate by density. Once separated, they are regenerated separately and then re-mixed. The amounts of the resins used must be chemically balanced, so that they both become exhausted at the same time.

Both the anion and cation resins may be either strong or weak-acting resins, but the usual (most economical) combination is a strong-acid cation resin paired with a weak-base anion resin. Since a weak-base anion resin cannot exchange the anions of weak acids unless the conditions are already acidic, the strong-acid cation resin in H+ form is always put first. To illustrate: a weak-base anion resin in OH- form will not exchange HCO3 – and CO3-2 ions unless there is free H+ already available to neutralize the OH. (This is analogous to the situation with WAC resin [see above, under Dealkalization]. which will not exchange H+ for hardness unless sufficient alkalinity is already available to balance it.) If the water encounters the cation resin loaded with H+ first, then the water will have exchanged all cations for H+ already and be strongly acidic when it enters the cartridge bed containing the weak-base anion resin.