MCM-41 mesoporous molecular sieves containing Nb, V, and Mo separately and together have been synthesised and characterised by high-resolution transmission microscopy (HRTEM with EDX), N-2 adsorption isotherms, and X-ray diffraction (XRD), and tested in the oxidative dehydrogenation (ODH) of propane in the gas phase and in the liquid-phase oxidation of cyclohexene with hydrogen peroxide. For the first time, it has been documented that the incorporation of niobium during the synthesis of MCM-41 materials results in the formation of defects, the presence of which influences the selectivity in both reactions, ODH of propane to propene and epoxidation of cyclohexene. It is proposed that defect holes, generated by niobium, are convenient for access of reagents to the active species and for making the diffusion easier (particularly important in the liquid-phase reactions). NbO- species plays the role of an electronic promoter in multimetallosilicates NbV- and NbVMoMCM-41, where vanadium is an active species. In the absence of vanadium, niobium species are active in both oxidation processes. The role of acidity in the catalysts used for both oxidation processes is considered. (C) 2004 Elsevier Inc. All rights reserved.