A critical evaluation and optimization of all available phase diagrams and thermodynamic data of the binary Sn–RE (RE=La, Ce, Pr, Nd and Sm) systems was conducted to obtain reliable thermodynamic functions of all the phases in the systems. In particular, the thermodynamic properties of liquid phase were described using the Modified Quasichemical Model which takes into account the short range ordering of solution. In the thermodynamic optimization, a systematic analysis involving the similarity and periodicity in the phase diagrams and the thermodynamic properties observed in the lanthanide series was applied to resolve inconsistencies in the available experimental data and to estimate the unknown thermodynamic properties and phase equilibria data.