Niobium-silicon mixed oxide nanocomposites, containing 7-37 wt.% Nb, were synthesized by a new sol-gel route and characterized for textural and acid properties by N-2 adsorption, NH3 TPD and FTIR of adsorbed acetonitrile. The materials were tested as catalysts for epoxidation of cyclooctene with H2O2. High surface areas (> 150 m(2) g(-1)), decreasing with Nb content, were found for materials containing up to 23.0 wt.% Nb. FTIR and TPD measurements evidenced the presence of both Bronsted and Lewis acid sites with different strength. The amount of strong acid sites, relative to those of low or moderate strength, increased with Nb content. The materials containing up to 23.0 wt.% Nb were active and stable catalysts for the epoxidation of cyclooctene. The highest activity and H2O2 Selectivity was found for the catalyst with the lowest No content, that showed also high stability in reuse. Catalytic properties were related to acid sites and to the presence of NbOx species with different coordination. (C) 2008 Elsevier B.V. All rights reserved.