The aim of the present work was to determine the structure of the quaternary alloy Zn 0.6 Mn 0.4 In 2 S 4 and to locate the Mn 2+ . This was accomplished by means of powder synchrotron X-ray diffraction, high-resolution microscopy and convergent-beam electron diffraction (CBED). The powder X-ray diffraction pattern was indexed in a rhombohedral cell, with cell constants a = 3.875 (2), c = 37.208 (4) Å, and possible space groups R \bar{3} m or R 3 m . Rietveld refinements using different cationic arrangements in these space groups were performed. A model in space group R 3 m , in which the tetrahedral and octahedral sites were occupied by different proportions of Zn, Mn and In atoms, gave the best result. The Rietveld refinement of this model led to figures of merit R wp = 9.8%, R p = 9.1% and χ 2 = 11.1. Selected-area electron diffraction patterns and high-resolution transmission electron micrographs along [001] reveal the rhombohedral configuration. CBED patterns perpendicular to [001], showing the distinctive 3 m symmetry, confirmed space group R 3 m and the breaking of the centrosymmetry of the parent compound, ZnIn 2 S 4 .