Two parameters have been added to the Extended UNIQUAC model of Thomsen and Rasmussen [Thomsen, K., Rasmussen, P., 1999. Modeling of vapor-liquid-solid equilibrium in gas-aqueous electrolyte systems. Chem. Eng. Sci. 54, 1787-1802] to account for the pressure dependency of mineral solubility. The improved model has been used for correlating and predicting vapor-liquid-solid equilibrium for different carbonate systems (CaCO3, MgCO3, BaCO3 and SrCO3) causing mineral scaling problems. The solubility of NaCl and CO2 in pure water and the solubility of CO2 in NaCl and Na2SO4 solutions have also been correlated. The results show that the Extended UNIQUAC model, with the added pressure parameters, is able to represent binary (NaCl-H2O, CaCO3-H2O, BaCO3-H2O, SrCO3-H2O, MgCO3-H2O, Mg(OH)(2)-H2O and CO2-H2O), ternary (CaCO3-CO2-H2O, BaCO3-CO2-H2O, SrCO3-CO2-H2O, MgCO3-CO2-H2O, CO2-NaCl-H2O and CO2-Na2SO4-H2O), and quaternary (CO2-NaCl-Na2SO4-H2O) solubility data within the experimental accuracy in the range of temperatures and pressures considered in the study, i.e. from 0 to 250 degrees C, and from 1 to 1000 bar, respectively. The modified Extended UNIQUAC model will be a useful tool for predicting and quantifying the scaling problems that may occur in wells and surface equipment during geothermal operations. This would allow adequate preventive measures to be taken before mineral deposition becomes troublesome. (c) 2006 CNR. Published by Elsevier Ltd. All rights reserved.