Abstract: The thermodynamic properties of aqueous NaCl solutions at high temperatures and pressures are important for many industrial and geological systems and also possess theoretical significance. It is shown that a very simple semiempirical equation can be used to represent the composition of aqueous sodium chloride solutions up to saturation over the range, 373-573 K and 0.1-100 MPa. This equation comprises a term of long-range electrostatic force derived from the well-known osmotic pressure equation by statistical mechanics, and another term of short-range intermolecular (or inter-ionic) force deduced from the non-electrolyte solution theory. The excess Gibbs energy of aqueous NaCl solutions can thus be expressed as follows:The activity coefficient of water and the mean ionic activity coefficient of NaCl are respectively. In the above equations, the pure liquid H2O and the supercooled liquid NaCl are taken as reference states, and the mole fraction on an ionized basis as the measure of composition. Only parameter w is freely adjustable, because all the other parameters are determined by the properties of pure water. A simple programmable calculator is adequate for the calculations. Agreement of the calculated osmotic coefficient (φ) with the experimental is excellent, the standard deviation of φ being ±0.0076 for the range 373-573 K at saturation pressure. According to the physical meaning of w illustrated by the Scatchard-Hildebrand theory, the large and negative value of w at low temperatures represents a dominance of ion hydration over the other inter-particle interactions. The Gibbs energy of fusion of NaCl in the range 373-573 K is obtained The calculated chemical potentials of NaCl in aqueous solutions with different reference states coincide with one another, showing the reliability of the equations.