The first peroxidovanadium trimer was prepared in the form of its tetrabutylammonium salt, (NBu4)(3)[V3O3(O-2)(6)]center dot 2H(2)O. Its X-ray structure analysis revealed a unique cyclic structure of the [V3O3(O-2)(6)](3-) ion incorporating the yet unobserved mu(3)-eta(2):eta(1):eta(1) coordination mode of one of its peroxido ligands. While relatively stable in nonaqueous solvents, the [V3O3(O-2)(6)](3-) ion quickly decomposes in diluted aqueous solutions. A. higher vanadium concentration or a higher CH3CN content in the mixed CH3CN/H2O solvent facilitates the formation of oligomers [V2O2(O-2)(4) (H2O)(2-) and [V3O3(O-2)(6)](3-)center dot V-51. NMR investigations indicated that the trinuclear species is incorporated in vanadium-catalyzed oxidations in the presence of H2O2.