Manganese-gallium samples with cation ratios Mn:Ga = 1:2, 1.5:1.5, and 2:1 are synthesized by coprecipitation with subsequent annealing in air in a temperature range 600–1200 °C. Powder XRD, TEM, and BET methods are used to study the physicochemical characteristics of the samples. It is found that in the air at the annealing temperature of 600 °C finely dispersed low-temperature Mn3–xGaxO4 spinels primarily form in all series, but in the whole temperature range (600–1200 °C) the system is multiphase. Annealing at 800–1200 °C leads to an increase in the concentration of simple oxides (β-Mn3O4 and β-Ga2O3). Only simple α-Mn2O3 and β-Ga2O3 oxides exist in a Mn:Ga = 2:1 series at 800 °C. In the sample with a cation ratio Mn:Ga = 1.5:1.5 annealed in air at 1000 °C, the formation of a superstructure based on the spinel structure is found.