A series of V- and Mo-V-based catalysts, i.e. undoped and K-doped VOx/Al2O3, MoVNbO, MoVSbO and MoVTeNbO mixed metal oxides, have been tested in the oxidation of propane and propylene. All the catalysts are active and relatively selective in the partial oxidation of propane and propylene, although the nature of reaction products changes depending on the characteristics of the catalysts. Thus, acrylic acid (MoVTeNbO), acetic and acrylic acid (MoVSbO), acetic acid (MoVNbO) and propylene and CO., (supported catalysts) were the most important products obtained front both propane and propylene. FT-IR spectroscopy of NH3 indicates the presence of Bronsted and Lewis acid sites, although the acidity decreases in the order: MoVNbO > VOx/Al2O3 > MoVSbO > MoVTeNbO > K-doped VOx/Al2O3. FT-IR studies of propylene adsorbed and desorbed at different temperatures allowed us to propose three different intermediates: (i) a pi-allylic compound, intermediate in the selective oxidation of propylene to acrylic acid; (ii) an enolic-type compound, intermediate in the hydration/oxidation of the olefin to form acetone and acetic acid; (iii) a pi-bonded propylene species interacting with Lewis acid sites, precursor in the deep oxidation of propylene. Accordingly, the key factors in the achievement of an active and selective catalyst for the oxidation of propane to acrylic acid as MoVTeNbO mixed oxides is tentatively proposed. The role of acid sites in selective and non-selective pathways are also discussed. (C) 2004 Elsevier B.V All rights reserved.