The structure of the thermoelectric Zn4Sb3 is refined using synchrotron X-ray powder diffraction data collected at wavelengths both near to and relatively far from the Zn adsorption edge. In agreement with earlier studies, the compound crystallized in a trigonal structure, space group R3̄c with a = 12.2406(3)Å, c = 12.4361(3)Å at room temperature, and there are three primary sites in the asymmetric unit. Each site contains only one atomic species, in contrast to many previous studies. The primary Zn (36f) site is slightly less than fully occupied, whereas the two Sb sites (18e and 12c) are fully occupied. In addition, several Zn interstitial sites (36f) with low occupancies (>5%) are also present. The results are in agreement with the model proposed by Snyder , as opposed to that originally proposed by Mayer  and more recently by Mozharivskyj . The refined site occupancies yield an overall stoichiometry which is consistent with that measured experimentally. The presence of interstitial Zn can be understood in terms of charge balance requirements and is likely responsible for the exceptionally low thermal conductivity of this material.