Fluid inclusion analyses reported in the literature, together with experimental data and thermodynamic calculations indicate that nonideality in the system H 2 O-CO 2 -NaCl may have a profound effect on phase relations in metamorphic processes. High concentrations of NaCl in the system H 2 O-CO 2 -NaCl increase substantially the size of the two-phase (liquid + vapor) region by raising the consolute temperature for a given pressure and ratio of the mole fractions of H 2 O and NaCl to more than double that in the binary system H 2 O-CO 2 . As a consequence, fluid immiscibility may occur at considerable depth during progressive metamorphism of siliceous carbonates. The effect of nonideal mixing of H 2 O, CO 2 and NaCl on equilibrium constraints in metamorphic systems can be assessed quantitatively on phase diagrams generated with the aid of a modified Redlich-Kwong equation of state (Bowers and Helgeson, 1983). Diagrams of this kind indicate that increasing NaCl concentration results in higher temperatures and/or lower predicted values of X co2 for equilibrium mineral assemblages than would be true for the binary system H 2 O-CO 2 . Transection of various mineral stability fields by saturation lines representing equilibrium between the aqueous phase and calcite, dolomite, or magnesite on logarithmic activity diagrams is also highly sensitive to the NaCl concentration in the aqueous phase.--Modified journal abstract.