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.