The ternary water + MgCl2 + CH4 phase equilibria were determined at the MgCl2 concentration range of 3 - 15 wt % and at temperature and pressure ranges of272 - 286 K and 30 - 130 bars, respectively. The ternary water + MgCl2 + CO2phase equilibria are also determined at the MgCl2 concenration range of 3 - 10wt % and at the temperature and pressure ranges of 272 - 282 K and 15 - 45bars, respectively. The quadruple points of the water + MgCl2 + CO2 system wherethe four phases, hydrate, water-rich liquid, CO2-rich liquid, and vapor phases, coexistwere also carefully measured and found to be located just below the vapor pressurecurve of pure carbon dioxide. For the prediction of hydrate phase equilibria, the vaporand liquid phases were treated with the Redlich-Kwong-Soave equation of state withthe Modified Huron-Vidal mixing rule and the hydrate phase with the van derWaals-Platteeuw model. The Pitzer-based model was adopted to estimate the activityof water in the aqueous electrolyte solution. This predictive approach was found to bewell applicable to hydrate systems containing various concentrations of electrolytes.