Surface structures of SiO2-supported niobium catalysts prepared by use of a Nb-monomer complex, a Nb-dimer complex, Nb pentaethoxides and Nb pentachlorides as precursors have been determined by means of EXAFS and XANES. These different Nb structures on SiO2 have been characterized by ethanol dehydrogenation as a test reaction. In contrast to the case of Mo catalysts, the activity and selectivity of the Nb catalysts dramatically depended on the structure of the reaction sites at an atomic scale. Dynamic structural changes of the Nb sites are presented and local structural changes at the interface between the supported Nb atom and the SiO2 surface are proposed to explain the role of support in catalysis.