The influence of mobium on the physicochemical properties of the Mo-V-O system and on its catalytic properties in the oxidation of ethane to ethylene and acetic acid is examined. Solids based on MoV0.4Ox, and MoV0.4Nb0.12Oy composition and calcined at 350 or 400 degrees C were studied by X-ray diffraction, and by laser Raman and X-ray photoelectron spectroscopies. Their reactivity during reduction and reoxidation was examined by in situ XRD and by XPS after pre-treatment. Their stability in air was evaluated by means of Raman spectroscopy during laser heating of particles. Niobium is responsible for both stabilization and nanosize of MoO3 and (VNbMo)(5)O-14 crystals. The high global selectivity to ethylene and acetic acid (90-96 mol%) is related to the presence of both phases while higher activity is owed to nanoparticles. The model already proposed by Merzouki et al. (Stud. Surf. Sci. Catal., 72 (1992) 81) suggesting that MoVNbO catalysts could be made up from (VNbMo)(5)O-14-type microdomains in MoO3 matrix seems still topical. (c) 2004 Elsevier B.V. All rights reserved.