Oxidative properties of MCM-41 molecular sieves containing niobium in the framework or/and in the extra framework positions were studied by means of H-2-TPR, FFIR + NO, ESR techniques and were tested in the oxidation of dibutyl sulphide with hydrogen peroxide. The application of various Nb-sources in the synthesis of NbMCM-41 materials allows the estimation of a role of oxalate ions in the generation of active centres. The activity of Nb-containing mesoporous molecular sieves in the oxidation of thioethers with H2O2 requires the presence of Nb-O- species in the framework. The growth of a concentration of niobium (from Si/Nb = 32 to Si/Nb = 16) results in the decrease of the number of active centres due to the release of oxygen during the activation procedure. Niobium species in the extra framework positions of MCM-41 catalysts are not active in the dibutyl sulphide oxidation with H2O2 unless they are leached to the solution. Oxidative centres on the catalyst surface cause the formation of nitrite/nitrate species after the adsorption of NO.