Single-crystal X-ray diffraction data, collected from a sartorite crystal from Lengenbach (Binntal, Valais, Switzerland), yielded a ninefold superstructure: space group P21/c; a = 37.71(2), b = 7.898(3), c = 20.106(8) Å, β= 101.993(7)°; R1 = 6.08% for 6293 reflections with I > 2σI. The sample is an N1,2 = 3,3 sartorite homologue and the refined formula Pb8Tl1.5As17.5S35 compares very well with the empirical formula Pb8.2Tl1.4As17.5Sb0.5S35 obtained from electron microprobe analyses. It has a high Tl content, up to 6.5 wt%. In a coupled substitution approximately 1.5 Tl+ replace about 0.5 As3+ and 1 PbS. The refined structure has 35 S atoms pfu instead of the expected 36 S (= 9 × 4 S, from PbAs2S4). The incorporation of substantial amounts of Tl+ into PbAs2S4 is essential for the type and periodicity of superstructures in sartorite. The refined superstructure can be interpreted as a so-called “lock-in” structure with a composition that yields a commensurate lattice for a mineral that usually has an incommensurate lattice. No commensurate periodicity could be found for a second crystal with about 0.5 Tl apfu.