Critical evaluation and optimization of all available phase diagram and thermodynamic data for the Fe-RE (RE = La, Ce, Pr, Nd and Sm) systems have been conducted. The thermodynamic properties of the liquid phase and solutions were described using the modified quasichemical model and the compound energy Formalism, respectively. The magnetic properties of solid phases were also taken into account. In order to obtain more accurate thermodynamic description, the systematic changes in the phase diagrams and liquid enthalpy of mixing along the lanthanide series were considered in the evaluation of experimental data.