The solubility of CaSO4 center dot 2H(2)O was determined in aqueous NaCl solutions up to very high salinities and at constant MgCl2 concentrations at 35 degrees C. The addition of MgCl2 into the aqueous NaCl system enhances the solubility of CaSO4 center dot 2H(2)O significantly at lower NaCl concentrations The solubility maximum of CaSO4 center dot 2H(2)O also shifts slightly toward the lower salinities with the addition of MgCl2 We also measured accurate density (p) and speed of sound (u) for the quaternary system CaSO4 center dot 2H(2)O + NaCl + MgCl2 + H2O at 35 degrees C. Measurements of the speed of sound and density have been used to determine solution isentropic compiessibility (kappa(S)) and ion-hydration characteristics Empirical equations describing the solubility, density, speed of sound, and isentropic compressibility in these systems arc presented