The study revealed peculiarities in the formation of mixed oxides with the Mn3-xAlxO4 spinel structure that were obtained from initial simple oxides β-Mn3O4 and γ-Al2O3 by calcination in various media at 600–1200 °C. The phase composition of the samples treated in air, argon or hydrogen was investigated using ex situ and in situ X-ray diffraction. It was shown that the occurrence of phase transformations depends on the treatment conditions and is related to the charge state of manganese cations. A scheme of the structural transformations induced by temperature and medium was proposed. The formation temperature of spinel was found to decreases from 950 to 600 °C when the oxidizing medium is replaced by the reducing one. Mn3-xAlxO4 spinel (x ∼ 1.5) was produced in air at temperatures above 1050 °C or in an inert medium at 800–1200 °C.