The structure of gamma-Al2O3, the most important catalyst support, is still under discussion; especially the positions of the Al3+ cations and vacancies are debated. One model proposes that gamma-Al2O3 consists of irregularly stacked nanocrystals with all Al3+ cations occupying spinel positions, another that some Al3+ cations occupy non-spinel positions, while a third model proposes that gamma-Al2O3 does not have a spinel structure. O-17 and Al-27 NMR measurements showed that 62.5-65% of the cations are at octahedral positions, in agreement with a spinel structure model, with DFT calculations, and with isostructural gamma-Fe2O3, but in disagreement with the non-spinel model. A reconsideration of the non-spinel model is, therefore, strongly recommended. In agreement with DFT calculations, TEM measurements showed that strong reconstruction of the surface of gamma-Al2O3 occurs. The (110) surface of gamma-Al2O3 is not atomically flat but forms nanoscale (111) facets, indicating that (100) and (111) facets have the lowest energy. (C) 2020 Elsevier Inc. All rights reserved.