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The Tl-Se system was investigated by DSC and X-ray diffraction measurements to clarify the contradictory information concerning the phase diagram reported in the literature. Besides the solution phase (gamma) previously called Tl₂Se but based on Tl₅Se₃ and the stoichiometric compound TlSe, a new phase Tl₂Se has been found to exist. Using the literature data and our experimental results we optimized the model parameters of the Gibbs-energies of all stable phases which resulted in an analytical description of the system. The liquid phase has been described with the association solution model, TlSe and Tl₂Se have been treated as stoichiometric while a sublattice model has been used for the description of the gamma phase. In spite of the presence of two miscibility gaps in the liquid, a fair agreement was obtained between experimental and calculated thermodynamic functions.