The silver bismuth tridecasulfide Ag_3.5Bi_7.5S ₁₃ crystallizes in the monoclinic space group C2/m. Its structure is built up of two alternating kinds of layered modules parallel to (001). In the module denoted A, octahedra around the metal positions (M = Ag/Bi, M2 and an S atom on 2/m, other atoms on m) alternate with paired monocapped trigonal prisms around Bi. The NaCl-type module B is composed of parallel eightmembered chains of edge-sharing octahedra running diagonally across it. Ag_3.5Bi _7.5S₁₃ is the member with N = 8 of the pavonite homologous series ^NP of ternary compounds with the general formula [Bi ₂S₃]₂·[AgBiS₂] _(N-1)/2.