The CaO-P2O5 binary system has been thermodynamically assessed using the CALculation of PHAse Diagram technique. A two-sublattice model, (Ca2+)(P) (O2-, PO3-, PO43-, PO7/22-, PO5/2)(Q), is proposed to describe the liquid phase in the system. The selection of the species constituting the liquid phase has been based on the structure of phosphate melts. All the solid phases have been treated as stoichiometric compounds. Taking into account the phase equilibrium data already measured and the thermodynamic properties determined, a set of thermodynamic functions have been optimized for the liquid and the CaP4O11, Ca2P6O17, and Ca-4(PO4)(2)O solid phases. A good agreement between calculated results and experimental data was obtained. The model proposed to describe the liquid phase offers a reasonable approximation to the thermodynamic properties of the system and could be used to attain a better understanding related to the behavior of melts in glass-forming systems.