The thermodynamic modelling of the Ba–Pb binary system was carried out with the help of CALPHAD (CALculation of PHAse Diagram) method. The liquid phase has been described with the association solution model with ‘ Ba 1 Pb 1 ’ associated complex. Ba 2 Pb,Ba 5 Pb 3 ,BaPb,αBa 3 Pb 5 _HT,βBa 3 Pb 5 _LT and BaPb 3 have been treated as stoichiometric compounds and a sublattice model has been used for the description of the bcc and fcc phases. Due to incompatible experimental phase diagram and thermodynamic data, two sets of thermodynamic parameters have been optimized and discussed. They differ mainly by the optimized value of the entropy of formation of the associated complex and by the entropies of the intermetallic compounds.