Phase equilibria in the quasi-ternary systems Tl2Se-HgSe-(DSe2)-Se-IV were investigated by XRD method. Isothermal sections of the systems at 520 K were constructed. Quaternary compounds (Tl2HgDSe4)-Se-IV were found at this temperature in both systems. They crystallize in the non-centrosymmetric tetragonal space group I (4) over bar 2m with the lattice parameters a = 0.80032(3) nm, c = 0.66879(4) nm (R-i = 0.0762, R-p = 0.1199) for Tl2HgSiSe4, and a = 0.80407(1) nm, c = 0.68852(2) nm (R-i = 0.0746, R-p = 0.1158) for Tl2HgSnSe4. (c) 2012 Elsevier Ltd. All rights reserved.