An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.5–1 mm diameter) micro beads, usually white or yellowish, fabricated from an organic polymer substrate. The trapping of ions occurs along with the accompanying release of other ions, and thus the process is called ion exchange.Most commercial resins are made of polystyrene sulfonate. Mixed bed resins or mixed bed ion exchange resins are mainly used in the water purification industry for polishing process water to achieve de mineralized water quality (such as after a reverse osmosis system).