A series of six bimetallic oxovanadium complexes (1–6; only one was purified) were investigated by electrospray quadrupole time-of-flight tandem mass spectrometry (ESI-QTOFMS/MS) in negative-ion mode. Radical molecular anions [M].- were observed in MS mode.Fragmentation patterns of [M].-were proposed, and elemental compositions of most of the product ions were confirmed on the basis of the high-resolution ESI-CID-MS/MS spectra. A complicated series of low-abundance product ions similar to electron impact (EI) ionization spectra indicated the radical character of the precursor ions. Fragment ions at m/z 214, 200, and 182 seem to be the characteristic ions of bimetallic oxovanadium complexes. These ions implied the presence of a V–O–V bridge bond, which might contribute to stabilization of the radical. To obtain more information for structural elucidation, three representative bimetallic oxovanadium complexes (1–3) were analyzed further by MS in positive-ion mode. Positive-ion ESI-MS produced adduct ions of [M + H]+, [M + Na]+, and [M + K]+. The fragmentation patterns of [M+Na]+ were different than those of radical molecular anions [M].- . Relatively simple fragmentation occurred for [M + Na]+ , possibly due to even-electron ion character. Negative-ion MS and MS/MS spectra of the hydrolysis product of Complex 1 supported these finding, in particular, the existence of a V–O–V bridge bond.