Abstract: Watanabeite Cu4(As,Sb)2S5, in which As>Sb, is a new copper sulphosalt that occurs with quartz in a hydrothermal vein at the Teine mine, Sapporo, Hokkaido, Japan. It is silvery lead-grey in colour with lead-grey streak. VHN100 = 253–306 kg/mm2, brittle. It has no cleavage and the fracture is uneven. The measured density = 4.66(2) g/cm3. The mean of six microprobe analyses gave Cu 41.1, Ag 0.1, Mn 0.3, As 15.4, Sb 14.3, Bi 2.4, S 26.2, a total of 99.8 wt.%, corresponding to: (Cu3.94Mn0.03-Ag0.01)Σ3.98 (As1.25Sb0.72Bi0.07)Σ2.04S4.98 (basis: total atoms = 11), or ideally, Cu4(As,Sb)2S5, with As > Sb. The X-ray powder pattern resembles that of tetrahedrite but has subsidiary diffractions and is similar to that of synthetic Cu24As12S31 (Maske and Skinner, 1971). It is indexed on an orthorhombic cell with a = 14.51 Å, b = 13.30 Å, c = 17.96 Å (all ±0.01 Å), and Z = 16. Calculated density is 4.66 g/cm3. It is optically similar to tetrahedrite but is grey and weakly bireflectant. No internal reflections were observed. The maximum and minimum reflectance values in air and in oil (nD = 1.515) for the COM wavelengths are: 470 nm−32.5, 31.5; 17.7, 17.0, 546 nm−32.0, 31.1; 17.0, 16.3, 589 nm−31.1, 30.3; 16.1 m 15.5, 650 nm−30.0, 29.3; 15.0, 14.5%, respectively. Watanabeite forms masses composed of aggregates of minute grains up to 50 µm in diameter. Apart from some minute inclusions of emplectite, native bismuth and tennantite, it is almost monominerallic.