Abstract A comprehensive thermodynamic model is developed for the HCl-H2O binary system based on the electrolyte nonrandom two-liquid activity coefficient model. Partial dissociation of electrolyte HCl to hydronium cation and chloride anion is incorporated to account for the true species in solution. Regressed from extensive thermodynamic data, the model parameters include molecule-molecule and molecule-electrolyte binary interactions parameters, as well as the chemical equilibrium constant parameters of the partial dissociation solution chemistry. The model accurately calculates all phase equilibria and thermodynamic properties including calorimetric properties with the concentration range from pure water to pure HCl and temperatures from 273 up to about 400 K.