A thermodynamic modeling of saline waters is achieved taking into account acid-base, ion associates formation and precipitation equilibrium. A microcomputer program, written in the graphical language LabView accomplished the necessities of iteration for the solution of simultaneous equations, calculations. Extended Debye-Huckel and Fitter models were used for the activity coefficient calculations. The program allows determining the repartition of fi ee ions, ion pairs, and weak acid species. Calcium carbonate stability of water was predicted thanks to calco-carbonic equilibrium curves calculation. Concentration factors for calcium carbonates, calcium sulfates and silica were also calculated. The calculations were carried out for different kinds of saline waters at different temperatures. For representative calculations, standard seawater and brackish feed water for a reverse osmosis desalting plant in Gabes region (in South Tunisia) were selected.