Rare earth oxide systems Al2O3-La2O3 and SiO2-La2O3 were thermodynamically assessed and optimized with the substitution model. The obtained model parameters and Gibbs free energy of tine compounds and pure components were applied to the prediction of the liquidus surface and isothermal sections of Al2O3-SiO2-La2O3 system Rare earth oxide systems Al2O3-La2O3 and SiO2-La2O3 were thermodynamically assessed and optimized with the substitution model. The obtained model parameters and Gibbs free energy of tine compounds and pure components were applied to the prediction of the liquidus surface and isothermal sections of Al2O3- SiO2-La2O3 system