The mixed MoO3-ZnO oxides with various mole fraction of Mo, X-Mo, obtained by the impregnation method were heated in air and in hydrogen. As for the mixed oxide heated in air, MoO3 reacted stoichiometrically with ZnO to give ZnMoO4 at X-Mo < 0.5, while at X-Mo > 0.5, the reaction did not proceed completely. On the other hand, for the mixed oxide heated in hydrogen, at X-Mo < 0.5 ZnMoO4 was reduced to ZnMoO3 with the anion vacancy, while at X-Mo > 0.5, MoO3 was reduced to MoO2 and ZnMoO4 was difficult to be reduced to ZnMoO3. The structure of the surface and/or near the surface was different from the bulk structure.