The crystal structure of a freibergite (Ag-tetrahedrite) from Keno Hill, Yukon, Canada, and a tetrahedrite from Turquetil Lake, NWT, Canada, have been refined. The freibergite, Cu_5.84Ag_4.22Fe_1.59Zn_0.43 Sb_4.10AS_0.13S₁₃, a = 10.610(2) Å, and the tetrahedrite, Cu_10.24Fe_1.03Zn_0.97Ag_0.05Sb_3.66As_0.28S₁₃, a = 10.364(2) Å, were both refined in space group I4¯3m. The refinements confirm that Ag does occupy the threefold coordinated metal site as first described by Kalbskopf (1972) and that Fe also occupies this site. The structural changes accompanying Ag substitution are described and a model to explain the upper limit of substitution at four atoms of silver for thirteen atoms of sulphur is proposed.