All the thermodynamic property data and phase diagrams related to solid and liquid P2O5 were critically evaluated and optimized to obtain the best set of Gibbs energies for all phases. Confusion in literature data due to the very sluggish nature of the reactions involving P2O5 was resolved. The Gibbs energies of O- and O′-P2O5 were evaluated for the first time based on the phase stability against gas. The Gibbs energy of liquid H-P2O5, liquid O-P2O5, and liquid O′-P2O5 were also properly evaluated for the first time based on the metastable and stable vapor pressures. The calculated triple points of H-P2O5, O-P2O5, and O′-P2O5 are 420°C and 4.600 bar, 575°C and 0.805 bar, and 580°C and 0.774 bar, respectively. The calculated polymorphic transition point between O-P2O5 and O′-P2O5 is located at 65°C and 0.001 bar.