A comparative study on the catalytic behaviour of Mo-V-Te-Nb-O and Mo-V-Te-O catalysts prepared from Te6+- or Te4+-containing precursors, i.e. H6TeO6 and TeO2, respectively, in the absence or in the presence of Nb oxalate and/or oxalic acid is presented. The characterization of catalysts by X-ray diffraction (XRD), diffuse reflectance and photoelectron spectra (XPS) before and after the calcination step suggests that the nature of metal precursors in the synthesis gel strongly modifies both the nature of crystalline phases and the oxidation state of reducible metals. The catalytic activity (and probably the incorporation of vanadium in the crystalline structures) is favoured by using a Te6+-containing precursor and Nb oxalate or a Te4+-containing precursor. However, Nb-containing catalysts present the better catalytic performance in the selective oxidation of propane to acrilic acid suggesting an important influence of the presence of Nb5+ cations in the catalysts on the formation and/or stability of acrylic acid. (c) 2004 Published by Elsevier B.V.