The thermodynamics of the U-Th-O system have been assessed using the Calphad method. The compound energy formalism ( CEF) and a partially ionic two-sublattice liquid model ( TSLM) were used for the fluorite U1-yThyO2 +/- x, c-(U,Th)(4)O-9, and the U-Th-O melt. The O-2 activity of fluorite U1-yThyO2-x with temperature and composition was determined by thermogravimetric analysis. Thermodynamic studies for the Th-O binary and U-Th-O ternary available in the open literature were critically reviewed. A self-consistent data set was selected and compiled with the equilibrium oxygen pressures determined by thermogravimetry in order to optimize the adjustable parameters of models selected to represent the phases in the Th-O and U-Th-O systems.