The phase equilibria in the quasi-ternary system Tl 2 S-HgS-SnS 2 at 520 K were investigated by X-ray diffraction. The isothermal section of the system at 520 K was constructed. One quaternary compound, Tl 2 HgSnS 4 , was found at this temperature. Tl 2 HgSnS 4 crystallizes in a tetragonal structure, noncentrosymmetric space group I-42m, unit cell parameters a = 7.8571(6) Å, c = 6.6989(7) Å, V = 413.5(1) Å 3 .