The atomic structure and the chemistry of basal-plane inversion boundaries in Sb2O3-doped ZnO were investigated using quantitative transmission electron microscopy techniques. Electron microdiffraction and high-resolution transmission electron microscopy were used to determine the orientation of the polar c-axis on both sides of the inversion boundary and the translation state between the inverted ZnO domains. Quantitative energy-dispersive X-ray spectroscopy combined with high-resolution transmission electron microscopy allowed us to determine the exact amount and the arrangement of antimony in the boundary layer. Inversion boundaries are head-to-head oriented with a displacement vector of the oxygen sublattice of R-IB = 1/3[01 (1) over bar0] - 0.102[0001]. The boundary plane consists of a highly ordered SbZn2 monolayer in which the cations occupy the octahedral interstices of the structure. In the octahedral boundary layer, zinc and antimony atoms constitute a honeycomb superstructure with a threefold (3m) in-plane symmetry.