We present a thermodynamic model for the aqueous Mg2+ - Na+ - Cl- quaternary system based on symmetric electrolyte Non-Random Two Liquid (eNRTL) theory. This work is a continuation of our previous study on the aqueous Ca2+ - Na+ - K+ Cl- quaternary system and part of a larger effort to develop a comprehensive engineering thermodynamic model for high salinity produced water in oil and gas production. The eNRTL theory requires two binary interaction parameters for each molecule molecule, molecule -electrolyte, and electrolyte -electrolyte pair to correlate composition dependence of the solution nonideality. The binary interaction parameters, including their temperature coefficients, are identified for the (Mg2+-Cl-):H2O pair, (Mg2+-Cl-):(K+-Cl-) pair, and (Mg2+-Cl-):(Na+-Cl-) pair using available thermodynamic data. The binary parameters for the other pairs are taken directly from the literature. Together, the eNRTL model and the binary parameters yield a satisfactory thermodynamic model for the quaternary system and its subsystems with temperatures ranging from 273.15 to 473.15 K and salt concentrations up to saturation. (C) 2017 Elsevier B.V. All rights reserved.