The paper presents an application of the one-fluid extended corresponding states method to the calculation of the thermodynamic surface of the ammonia–water mixture. Each pure component of the mixture is considered as a reference fluid, and to test the behavior of the model in wide ranges of temperature and pressure, the Haar–Gallagher and the Pruß–Wagner equations of state were chosen for pure ammonia and water, respectively. To avoid numerical problems during the calculation of the pure-component equivalent substance reducing ratios (or scaling factors), a method based on the mapping defined by the extended corresponding states algorithm and two-dimensional interpolation is proposed. The estimation of the binary interaction parameters was performed using the general case of the least squares method, i.e., the case when all measurements (observations) and unknowns are subject to uncertainty and are adjusted simultaneously with the constraint equations. The results show a strong temperature and composition dependence of both interaction parameters for the liquid, as well as for the vapor phase. A formulation for the binary interaction parameters as continuous functions of these variables was also derived and optimized using the structural optimization and regression analysis. The final statistical quality of the approach presented in the paper was assessed using the available data on thermophysical properties of the mixture.