The Fe–Al end-member spinel hercynite was prepared by the arc plasma synthesis (APS) technique and subjected to annealing at 1273 K for 1, 24 and 36 h in air atmosphere. The X-ray diffraction (XRD) analyses conducted before and after the thermal treatment revealed an evident alteration of the original hercynite. About half of the original spinel is preserved after the longest annealing experiment (36 h), the reminder of the Fe–Al spinel underwent decomposition into iron and aluminum oxide phases, among which non-stoichiometric magnetite and corundum were the most important. Mössbauer spectroscopy revealed that the initial hercynite was a normal spinel with a slight inversion degree of 11 %, which was increased to 34 % after the 36-h annealing process. An overlapping of reflections for spinel-like phases γFe2O3 and γAl2O3 with Fe–Al spinel impeded their detection by XRD. Mössbauer spectroscopy (MS) confirmed the presence of magnetite in the annealed samples. The SEM-EPMA observations of the oxidized surfaces of hercynite showed well-developed, scalenohedral crystals with different Fe/O ratios, and well defined compositions corresponding to Al-bearing Fe2O3 as measured by WDS spectroscopy. The results of XRD, MS and SEM-EPMA permit to establish a general oxidation mechanism of hercynite decomposition.