Using classical methods, differential thermal analysis (DTA) and X-ray powder diffraction at different temperatures, the Tl2SO4-Li2SO4 phase diagram has been re-examined. Several changes were observed from a previous study. Two small domains of primary solid solution with beta-Li2SO4 (low-temperature form) and alpha-Tl2SO4 (high-temperature form) structures have been found, along with the beta(2)-Tl2SO4<->alpha-Tl2SO4 transition. The mixed sulfate TlLiSO4 exists under two forms. The low-temperature form beta is orthorhombic with a = 8.3815(4) Angstrom, b = 9.2914(4) Angstrom and c = 5.3175(3) Angstrom; the space group is Pnam or Pna2(1), with Z = 4. The high-temperature form a is hexagonal with a = 18.6699(8) Angstrom and c = 8.7015(6) Angstrom; the space group is P6(3), with Z = 24. The X-ray powder patterns are given. The structures of these two varieties belong to the stuffed-tridymite type. The beta<->alpha transition occurs at 531 degrees C with a modification of the structural skeleton topology and an increase ( x 120) of the electrical conductivity which leads to classify TlLiSO4, as a superionic conductor.