Thermodynamic modeling coupled with key phase diagram experiments of the ternary CaO-SiO<sub>2sub>-ZrO<sub>2sub> system was carried out. The isothermal phase diagram of the CaO-SiO<sub>2sub>-ZrO<sub>2sub> system at 1873 K was established by using a classical phase equilibration and quenching technique followed by EPMA phase analysis. The Gibbs energy of each phase was optimized through critical evaluation of all available thermodynamic properties and phase diagram data in literature with new experimental data. The discrepancies between experimental phase diagram data and possible errors in the existing thermodynamic data were resolved in this study.