Strong acid cation (SAC) exchange resin is one of the most widely used ion exchange resins, which is composed of a polymer matrix and strong acid groups (-SO3H, -CH2SO3H, etc.) attached to the matrix. SAC resins with strong acid functionality can exchange positive ions and are used extensively for water purification, water softening, purification of pharmaceuticals and food, and catalysis.

For softening applications, SAC resins are very effective at complete removal of hardness ions such as magnesium (Mg2+) or calcium (Ca2+) and are regenerated with either a sodium salt solution. For demineralization applications, SAC resins can be regenerated with acid to convert the resins to their hydrogen (H+) form. SAC resins may also be used in split-stream dealkalization, where two SAC beds operate in parallel, the first as a softener that produces an alkaline solution, and the second as a demineralizer, resulting in alkalinity being removed from the blended stream. Additionally, SAC resins have been developed for the removal of barium (Ba) and radium (Ra) from drinking water or other streams.