Rietveld refinement of neutron powder diffraction data on four samples of synthetic, iron-bearing tetrahedrite (Cu12-xFexSb4S13) with x = 0.28, 0.69, 0.91, 2.19 and four samples of synthetic tennantite (Cu12-xFexAs4S13) with x = 0.33, 0.38, 0.86, 1.5 indicate unambiguously that iron is incorporated into tetrahedral M1 (12d) sites and not into triangular M2 (12e) sites in the cubic crystal structure (space group I (4) over bar 3 m). The refinement results also confirm that M2 is a split (24g), flat-pyramidal site situated statistically on both sides of the S1-S1-S2 triangle. In tetrahedrite, this split is about 0.6 angstrom, in tennantite about 0.7 angstrom. Trends in bond lengths and magnitude of the M2 split were evaluated by means of linear regression with Fe concentration as the independent variable.