The Nb dimers attached on SiO2 were prepared by using the dimeric complex [Nb(eta(5)-C5H5)H-mu-(eta(5),eta(1)- C5H4)](2) as precursor. The surface Nb structure was characterized by EXAFS and XANES to possess the monomer-pair structure without Nb-Nb bonding. It was suggested that the catalytic ethanol dehydration on the monomer-pair catalyst proceeds in conjunction with the structure change of the Nb sites involving a formation-break cycle of Nb-Nb bonds, that is, a reversible monomer-dimer transformation. The catalytic performance is compared with that of the Nb dimers on SiO2 previously reported.