In this work we report the results of experimental studies of the phase composition and structure of the nanodispersed Al–O powder produced by the plasmodynamic method. Using X-ray diffractometry and transmission electron microscopy, we show that the product contains σ-Al2O3 and MgAl2O4. The phase composition evolution of the initial product of plasmodynamic synthesis is studied during its heat treatment. Thermogravimetric analysis in an argon atmosphere shows no changes in mass of the sample weighed when it is heated, while differential scanning calorimetry studies indicate a marked manifestation of the endothermic effect.