Abstract Using the CALPHAD technique, the phase diagram and thermodynamic data of the Mg–Tl system are critically reevaluated. The liquid, hcp and bcc phases are treated with the substitutional solution model, where the excess Gibbs free energies are expressed in Redlich–Kister polynomials. All the intermetallic phases are treated as stoichiometric compounds. A self-consistent thermodynamic description of the Mg–Tl system is obtained. The calculated phase diagram and thermodynamic properties are in satisfactory agreement with the experimental results.