The crystal structure of allochalcoselite, Cu+Cu2+5PbO2 (SeO3)2Cl5, a new mineral species from the fumaroles of the Great Tolbachik fissure eruption, in the Kamchatka Peninsula, Russia, has been solved from single-crystal X-ray-diffraction data by direct methods and refined to R1 = 0.047 for 1235 unique reflections with Fo ≥ 4σF. The mineral is monoclinic, space group C2/m, a 18.468(2), b 6.1475(8), c 15.314(2) Å, β 119.284(2)°, V 1516.5(3) Å 3, Z = 4. The structure contains one symmetrically unique Pb position coordinated by three O and four Cl atoms. There are two Se4+ cations, which form SeO3 trigonal pyramids. There are six symmetrically independent Cu sites in allochalcoselite, four of which [Cu1-Cu4] are occupied by Cu2+ and two [Cu5 and Cu6] by Cu+ cations. The Cu1, Cu2, and Cu3 sites have distorted octahedral [4O + 2Cl], [(3O + Cl) + 2Cl], and [(3O + Cl) + 2Cl] coordinations, respectively. The Cu4 site forms a trigonal CuO3Cl2 bipyramid with one equatorial and one apical vertex occupied by Cl. The Cu+