The stability of spinel-type mixed Mn1.5Ga1.5O4 oxide prepared in an inert medium (1000 °C, Ar) is studied by thermogravimetry and high-temperature X-ray diffraction in air in a wide temperature range 30–1000 °C. On heating, reversible decomposition processes of initial spinel are observed. From 30 °C to 600 °C oxygen atoms attach to the surface layer of initial Mn1.5Ga1.5O4 spinel to form a new phase distinct from parent oxide by the oxygen stoichiometry (cation vacancies are formed). The product of decomposition is two oxides: Mn1.5Ga1.5O4 and Mn1.5–xGa1.5–x[·]xO4. On the contrary, above 600 °C a loss of oxygen occurs, the concentration of cation vacancies decreases in Mn1.5–xGa1.5–x[·]xO4, and the reverse process of single phase oxide crystallization takes place. At 1000 °C the spinel phase forms again whose composition is similar to that of the initial parent phase Mn1.5Ga1.5O4. On cooling the decomposition of this phase is again observed due to oxygen attachment.