Manganese-gallium oxide samples with the cationic ratio Mn:Ga = 2:1, 1.7:1.3, 1.5:1.5, 1.3:1.7 and 1:2 were synthesized via coprecipitation followed by calcination in argon over a wide temperature range, 600–1200 °C. Physicochemical characteristics of the samples were examined by means of XRD, BET, TG with mass spectroscopy, and IR spectroscopy. The XRD, IR spectroscopy showed that the Mn3-хGaхO4 mixed oxides with the spinel structure are formed in the entire temperature range, 600–1200 °C. Samples with a cation ratio Mn:Ga = 1.7:1.3, 1.5:1.5 at 1000–1200°С and Mn:Ga = 1.3:1.7 at 1200°С are single-phase ones according to XRD. The cation distribution over tetrahedral and octahedral spinel positions was obtained by the Rietveld method. The spinel phases synthesized at 600 °C (low-temperature samples) are nanocrystalline and have CSR sizes of 10–15 nm. Ex situ and in situ XRD, TG with mass spectroscopy studies revealed that low- and high-temperature Mn3-хGaхO4 oxides substantially differ from each other. Strongly differing unit cell parameters of the spinel phase in low- (600–800 °C) and high-temperature (1000–1200 °C) samples were revealed. The presence of excess oxygen in low-temperature Mn3-хGaхO4 phases was supposed. In situ XRD in flowing helium and TG with mass spectroscopy demonstrated a two-step loss of oxygen.