A new general model that describes the thermodynamic properties ofMgCl2(aq)has been developed from a global fit to experimental results, including isopiestic molalities, vapor pressure measurements, freezing-point depressions, enthalpies of dilution, heat capacities, and densities, for this system. The model is based on a recent ion-interaction treatment with extended higher-order virial terms, and on experimental results from 240 to 627 K at pressures to 100 MPa and molalities to25 mol⋅kg−1.©1998 American Institute of Physics and American Chemical Society.