VMoNbTeO catalysts show high activity in the reactions of selective oxidation of ethane and propane. Time-resolved X-ray diffraction is used to study structural transformations of a mixed oxide with the composition (TeO)0.18(Mo0.7V0.22Nb0.08)20O56 and the structure of the active M1 phase upon heating in a hydrogen atmosphere (1% H2 in helium). It is shown that the structure of the M1 phase is destroyed in the presence of a reducing agent above 480 °C. The process is accompanied by structural distortions, diminishing of the size of crystalline blocks, elimination of tellurium (due to the reduction and sublimation), and the reduction by molybdenum and vanadium cations. As a result, VMoNb oxides with a disordered structure are formed and rapidly crystallized at T > 480 °C into the rutile-type monoclinic oxide (Mo,V,Nb)O2 with a variable composition. X-ray photoelectron spectroscopy and transmission electron microscopy data confirm heterogeneous compositions of oxides formed by the decomposition of the initial phase. The surface of the formed particles is enriched with vanadium and niobium present in the composition of the oxides with disordered structure.