Two additional parameters to account for the pressure dependency of solubility are added to the Extended UNIQUAC model presented by Thomsen and Rasmussen (1999). The improved model has been used for correlation and prediction of vapor-liquid-solid equilibrium for different carbonate systems (CaCO3, MgCO3, BaCO3 and SrCO3) causing scale problems. The solubility of NaCl and CO2 in pure water, and the solubility of CO2 in solutions of different salts (NaCl and Na2SO4) have also been correlated. The temperature and pressure range covered is from 0 to 250°C and from 1 to 1000 bar, respectively. The results show that the Extended UNIQUAC model, with the proposed pressure parameters, is able to represent binary (NaCl-H2O, MCO3-H2O and CO2-H2O), ternary (MCO3-CO2-H2O, CO2-NaCl-H2O and CO2-Na2SO4-H2O) and quaternary (CO2-NaCl-Na2SO4-H2O) solubility data within the experimental accuracy in the temperature range from 0 to 250°C, and the pressure range from 1 to 1000 bar. M stands for Ca2+, Mg2+, Ba2+ and Sr2+.