The thermodynamic optimization of the Ce–La and Ce–La–Mg systems has been carried out using the CALPHAD technique. The solution phases (liquid, bcc, fcc, hcp and dhcp) were described with a substitutional solution model. The isostructural compounds MgCe in the Ce–Mg system and MgLa in the La–Mg system with B2 structure form a continuous range of solid solution in the Ce–La–Mg system. The order–disorder transition between the solutions with A2 structure and compounds with B2 structure in the system has been taken into account and thermodynamically modeled. The other isostructural compounds Mg 12 Ce and Mg 12 La, Mg 17 Ce 2 and Mg 17 La 2 , Mg 41 Ce 5 and Mg 41 La 5 , Mg 3 Ce and Mg 3 La, and Mg 2 Ce and Mg 2 La were described as such formula Mg 12 (Ce, La), Mg 17 (Ce, La) 2 , Mg 41 (Ce, La) 5 , Mg 3 (Ce, La, Mg), Mg 2 (Ce, La), respectively. A set of self-consistent thermodynamic description of the Ce–La–Mg system was obtained.