The oil and gas production from deepwater has been challenged due to limited knowledge of mineral solubility and inhibitor efficiency at the extreme conditions of ultrahigh temperature, pressure, and TDS (total dissolved solids) contributed by mixed salts. The solubility of calcite (CaCO3) was measured for temperatures up to 250 degrees C and pressures up to 1450 bar (21 000 psi) in the presence of mixed electrolytes. A set of stability constants was selected based upon experimental measurements, which, together with the current set of Pitzer coefficients for activity coefficients, produce predictions consistent with measured data for the NaCl system. Predictions for the system with high concentrations of mixed electrolytes, however, demonstrate deviation from the measurements due to the presence of SO42-. A need is therefore suggested to re-evaluate the pressure dependence of Pitzer coefficients corresponding to the interactions between various key species in the calcite system and SO42-.