Single crystals of two new mercury thiohalides of the composition Hg3S2Cl2− x Brx(x = 0.5) have been grown from gas phase and studied by X-ray crystallography. Structure refinement for monoclinic (I) and cubic (II) phases (I: a = 16.841(2) Å, b = 9.128(2) Å, c = 9.435(4) Å; β = 90.080(10)°, V = 1450.3(7) Å3, space group C2/m, Z = 8, R = 0.0528; II: a = 18.006(2) Å, V = 5837.8(11) Å3, space group $$Pm\bar 3n$$ , Z = 32, R = 0.0503) clearly shows that they are polymorphs of the same composition Hg3S2Cl1.5Br0.5. The monoclinic modification I is similar to the synthetic phases γ-Hg3S2Cl2, β-Hg3S2Br2, Hg3Se2Br2 and to the analogue of radtkeite mineral, Hg3S2ClI. The modification II is isostructural to the synthetic β-Hg3S2Cl2. In both structures, each S atom coordinates three Hg atoms with the formation of pyramidal SHg3 units (Hg-S 2.37–2.48 Å; HgSHg 93.1–97.5 ). The SHg3 units are linked through Hg vertices into corrugated layers [Hg12S8]∞∞ (I) and isolated cubic groups [Hg12S8] (II). Similarly to other mercury chalcohalides, the crystal structures are basically determined by the halogen atoms which form a cubic sublattice incorporating the Hg-S moieties.