Spinel-type Ga2O3-Al2O3-ZnO mixed oxides were prepared by the glycothermal reaction of zinc acetylacetonate hydrate, gallium acetylacetonate, and aluminum isopropoxide. With increasing Zn content, the crystallinity of the spinel structure of the glycothermal product increased, although excess Zn content resulted in the formation of zinc oxide as a byproduct. To optimize the Ga/Al/Zn ratio, the activities of catalysts prepared from various starting compositions and calcined at 700 degrees C were tested for the methane-SCR of NO under wet conditions. For this reaction, catalysts with compositions in a narrow range exhibited high activity. An increase in crystallinity of the catalyst decreased the amount of adsorbed water on the surface of the catalyst and improved the tolerance for water present in the feed gas, which inhibited the catalytic activity for the methane-SCR of NO. The optimal ratio for the spinel-type Ga2O3-Al2O3-ZnO catalyst was Ga/Al/Zn = 1/5/0.75. This catalyst exhibited a higher activity than gamma-Ga2O3-Al2O3 binary catalyst for the methane-SCR of NO under wet conditions.