The vanadium—molybdenum oxide (VMoO) catalyst has been studied by X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis. (TGA). The results show that the surface composition of VMoO catalyst strongly depends on temperature. At room temperature, the catalyst surface is usually enriched with molybdenum. When the sample is heated to above 350–400°C, the vanadium species diffuse from bulk to surface; with a rise in temperature, the ratio of vanadium-to-molybdenum gradually increases and the surface even becomes enriched with vanadium. Besides, when the VMoO catalyst is heated to a certain temperature, the sample can absorb or evolve oxygen according to the ambient oxygen partial pressure. During the redox process, the vanadium species in VMoO catalyst are liable to change their chemical state, the V 4+ and V 5+ sites seem to act as the inlet and outlet sites, respectively of oxygen. The function of molybdenum species in VMoO catalyst seems to be to promote the evolution of lattice oxygen and tend to retain the ratio of V 4+-to-V 5+ at a certain value, which may be important for high selectivity and activity of the VMoO catalyst.