The binary BaO-CaO and BaO-SiO2 systems have been critically evaluated based upon available phase equilibrium and thermodynamic data and optimized model parameters have been obtained giving the Gibbs energies of all phases as functions of temperature and composition. The liquid solution has been modeled with the Modified Quasichemical Model (MQM) to account for the short-range ordering. The results have been combined with those of previous optimizations of the CaO-SiO2 system to optimize the BaO-CaO-SiO2 system.