In order to enable efficient recycling of rare earth metals through high temperature processes, the phase relations in rare earth oxide containing slags need to be understood. A thermodynamic description of the CaO-SiO2-Nd2O3 ternary system and its Nd-containing binary subsystems was, therefore, constructed in this study, based on existing phase diagram and thermodynamic data. The Gibbs energy of the liquid oxides in the CaO-SiO2-Nd2O3 system was modeled using the Modified Quasichemical Model. The solid phases Ca2-2xNd2xSiO4Ox, Ca3-3xNd2xSiO5, and Ca2+10xNd8-8x(SiO4)6O2-2x were modeled with polynomial expansions of the excess Gibbs energy. The optimized phase diagram of the CaO-SiO2-Nd2O3 system is in good agreement with the experimental data.