A computer program was developed to calculate equilibrium solution compositions in calcareous and gypsiferous systems. The calculations in the program are based upon published solubility products of calcite and gypsum and take into account ion-pair formation and the solubility enhancement of calcite resulting from the presence of Mg and SO4 in the solution. For a given pre-equilibrium solution, the program predicts equilibrium ionic strength, ionic activity coefficients, dissociated (free) ions, ion-pairs, and total concentration of solution species, amount of CaCO3 precipitated or dissolved at a specified partial CO2 pressure, and the amount of gypsum precipitated. Lime and gypsum solubility results predicted with the computer closely agree with published experimental results. The program is suitable for assessing the sodium and salinity hazards of marginal quality waters using other independently developed parameters, such as the concentrating effects taking place in the soil solution due to evapotranspiration during the cropping cycle and partial CO2 pressures developed under particular soil and cropping conditions.