SiO2-supported clusters of tantalum were synthesized from adsorbed Ta(CH2Ph)(5) by treatment in H-2 at 523 K. The surface species were characterized by X-ray absorption spectroscopy ( extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES)) and ultraviolet-visible spectroscopy. The EXAFS data show that SiO2-supported tantalum clusters were characterized by a Ta-Ta coordination number of approximately 2, consistent with the presence of tritantalum clusters, on average. When these were reduced in H-2 and reoxidized in O-2, the cluster nuclearity remained essentially unchanged, although reduction and oxidation occurred, respectively, as shown by XANES and UV-vis spectra; in the reoxidation, the tantalum oxidation state change was approximately two electronic charges per tritantalum cluster. The data demonstrate an analogy between the chemistry of group 5 metals on the SiO2 support and their chemistry in solution, as determined by the group of Cotton.