The solid solubilities of Al in Mg12La, Mg12Ce, Mg12Pr, Mg41Nd5 phases were investigated by key experiments. The enthalpy of formation of ternary compounds was estimated using the model of Miedema. The systematic thermodynamic evaluation and optimizations of ternary Al-Mg-RE (RE = La, Ce, Pr, Nd, Sm) systems were based on the available experimental data and current investigation. The Al-Mg-Pr and Al-Mg-Nd systems were optimized for the first time using a systematic approach. Arc-melted key alloys were annealed at T = 673 K for four weeks and the phases were analyzed with electron probe micro-analysis (EPMA). The Mg12La intermetallic phase was confirmed by the present investigation. Optimized model parameters have been obtained for the Gibbs energy functions of all stable phases, and the model reproduces most critically assessed experimental results. The Modified Quasichemical Model, which takes short-range ordering into account, is used for the liquid phase and the Compound Energy Formalism (CEF) is used for the solid solutions in the binary and ternary systems. Kohler-type extrapolation from binary systems was employed, where no ternary parameters were used for the liquid phase. (C) 2012 Elsevier Ltd. All rights reserved.