Thermodynamic optimizations of unary ZrO2, binary CaO-ZrO2 and SiO2-ZrO2 systems were carried out based on the critical evaluation of all available thermodynamic properties and phase diagram data in literature. Liquid phase was described by using the Modified Quasichemical Model considering the short range ordering, and solid solutions were described considering their defect structures. The discrepancies among experimental phase diagram data and possible errors in the existing thermodynamic data were identified and the most reliable thermodynamic descriptions of the systems were obtained.