The ternary water + MgCl2 + CH4 phase equilibria were determined at the MgCl2 concentration ranges of 3-15 wt.% and at temperature and pressure ranges of 272-286 K and 30-130 bars, respectively. The ternary water + MgCl2 + CO2 phase equilibria are also determined at the MgCl2 concentration ranges of 3-10 wt.% and at the temperature and pressure ranges of 272-282 K and 15-45 bars, respectively. The quadruple points of the water + MgCl2 + CO2 system where the four phases, hydrate, water-rich liquid, CO2-rich liquid, and vapor phases, coexist were also carefully measured and found to be located just below the vapor pressure curve of pure carbon dioxide. For the prediction of hydrate phase equilibria, the vapor and liquid phases were treated with the Redlich-Kwong-Soave equation of state with the Modified Huron-Vidal second order mixing rule and the hydrate phase with the van der Waals-Platteeuw model. The Fitter-based model was adopted to estimate the activity of water in the aqueous electrolyte solution. This predictive approach was found to be well applicable to hydrate systems containing various concentrations of electrolytes. (C) 1998 Elsevier Science B.V. All rights reserved.