The experimental study of e.m.f.s in {xZnO + (1 - x)P2O5}, (0.35 < x < 0.75), melts in the temperature region 973 K to 1173 K was carried out. On the basis of experimental e.m.fs the chemical potential, partial molar entropy, and partial molar enthalpy of ZnO and P2O5 referred to corresponding molar functions for the pure oxide, and integral functions of formation of the melts studied, were calculated. Using the model of ideal-associated solutions the chemical potentials of ZnO and P2O5 at the temperatures 973 K and 1173 K were simulated. Also the equilibrium constants of ZnO.P2O5, 2ZnO.P2O5, and 3ZnO.P2O5 were calculated. The experimental and simulated results show good agreement.