The inhibition of calcium carbonate precipitation from calco-carbonic waters equivalents to those used in the desalination plant is studied by means of two accelerated scaling methods. Mineral (sodium tripolyphosphate and sodium hexametaphosphate) and organic (sodium polyacrylate and sodium polyacrylate, RPI2000) inhibitors were tested. The phenomenon of limits efficiency, which presents the minimal inhibitor concentration, is observed with all tested inhibitors and with the two techniques. Polyacrylate inhibitor, RPI2000, presents a solution of the environmental problem met when phosphate inhibitors are used in the reverse osmosis (RO) unit. Pretreatment of geothermal waters destined to Gabes (Tunisia) plant desalination by RO is studied using a decarbonation reactor. The softening process of the waters is performed during air bubbling in an open double partition reactor by seeding of calcium carbonate recovered on site constituents. The water is treated in continuous flow without chemical additions, so the increase of pH is minimized. Consequently, there is no Mg(OH)(2) precipitation that produces suspended micro flocs, and we can avoid RO membrane fouling.