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Abstract: Realistic simulations of fluid flow in geologic systems have severely been hampered by the lack of a consistent formulation for fluid properties for binary salt–water fluids over the temperature–pressure–composition ranges encountered in the Earth’s crust. As the first of two companion studies, a set of correlations describing the phase stability relations in the system H2O–NaCl is developed. Pure water is described by the IAPS-84 equation of state. New correlations comprise the vapor pressure of halite and molten NaCl, the NaCl melting curve, the composition of halite-saturated liquid and vapor, the pressure of vapor+liquid+halite coexistence, the temperature–pressure and temperature–composition relations for the critical curve, and the compositions of liquid and vapor on the vapor+liquid coexistence surface. The correlations yield accurate values for temperatures from 0 to 1000°C, pressures from 0 to 5000bar, and compositions from 0 to 1 X NaCl (mole fraction of NaCl). To facilitate their use in fluid flow simulations, the correlations are entirely formulated as functions of temperature, pressure and composition. [Copyright &y& Elsevier]