Results are reported concerning the synergetic effect observed in the oxidation of propane to acrylic acid over the orthorhonribic M1 and hexagonal M2 phases present in the most active and selective MoVTe(Sb)NbO catalysts. The pure phases and phase mixtures containing either tellurium or antimony have been prepared and individually tested as catalysts. The result, obtained confirm that the phase responsible for the catalytic properties of the efficient catalysts is phase M1, and that M2 is poorly active. Mechanical mixtures of the pure phases have also been prepared and tested. All of the catalysts have been characterized before and after the catalytic reaction by X-ray diffraction. X-ray photoelectron spectroscopy, and high-resolution electron microscopy with EDS analyses. Although the synergetic effect previously described (bifunctional catalysis with the oxidative dehydrogenation of propane molecules on the orthorhombic M1 phase and the subsequent oxidation of propene on the hexagonal M2 phase) was observed, another cause related to migration of tellurium from the M2 phase to file surface of the M1 phase was indicated. This migration should balance a loss of tellurium in the active phase occuring under the conditions of a catalytic test, but may also create new dehydrogenation sites for propene and/or anneal total oxidation sites, The hexagonal M2 phase would thus play a role of tellurium reservoir for the active M1 phase. This effect was not reversible and concerned Only the tellurium. Antimony, which is less volatile, should not be lost by the active phase. Furthermore, it Was shown not to diffuse at the surface of the phases. (c) 2004 Elsevier Inc. All rights reserved.