A structural investigation of dufrenoysite has been carried out. There are two chemical units of Pb8As8S20 in a unit cell. The space group is C22–P21, and the unit-cell constants are a = 7.90 Å, b = 25.74 Å, c = 8.37 Å, β = 90°21′.The main structure is the same as that of rathite-I, rathite-Ia and rathite-III, the differences among them lying in the chemical composition and in small shifts of atoms due to the difference in composition. Each of four out of eight independent Pb atoms is surrounded by nine S atoms, the other four Pb atoms each being surrounded by seven (6 + 1) S atoms. The seventh of the nearest S atoms of the latter type is at a fairly distant position. All As atoms seem to have trigonal pyramidal coordinations of S atoms. Seven independent AsS3 pyramids are joined by sharing S atoms, giving an As4S9 and an As3S7 group. The remaining AsS3 pyramid is probably isolated.