The dioxovanadium(IV) complexes with pida(4-) ligands (H(4)pida) = N-(phosphonomethyl)iminodiacetic acid), Na-4[V2O2{(O)(2)P(O)CH2N(CH2COO)(2)}(2)]. 10H(2)O (1) and Na-8[V2O2{(O)(2)P(O)CH2N(CH2COO)(2)}(2)](2). 16H(2)O (2), were isolated from reactions of H(4)pida with either oxovanadium(V) (i.e., NaVO3) or oxovanadium(IV) precursors within the pH range of 2-8. The structures of complexes 1 and 2 were investigated by X-ray diffraction methods and in contrast to expectation were both found to be dinuclear. Complex I crystallized in the monoclinic system : P2(1)/n, a = 10.5632(1) Angstrom, b = 11.1868(1) Angstrom, c = 12.6921(1) Angstrom, beta = 106.45 degrees, V = 1438.44(2) Angstrom(3), Z = 4, and R (wR2) 0.0781 (0.2017). Complex 2 crystallized in the monoclinic system P2(1)/c : a = 13.9822(2) Angstrom; b = 11.1888-(2) Angstrom, c = 18.6519(3) Angstrom, beta = 100.88 degrees, V = 2865.51(8) Angstrom(3), Z = 4, and R (wR2) = 0.046 (0.125). Both complexes I and 2 have similar dimeric frameworks where two vanadium centers are linked by two phosphonate groups of two pida4-ligands (quadridentate binucleating), bridging through their four oxygen atoms to form a V2O4P2 eight-membered ring which possesses a crystallographic inversion center. In contrast to their solid-state features, in aqueous solution both dinuclear crystalline compounds immediately dissociate to monomeric species, as observed by EPR and UV/vis spectroscopy. Both solution-state EPR and NMR spectroscopy confirmed that redox chemistry is involved in the reaction between vanadate and H(4)pida. Studies in mixed solvent systems showed that the dinuclear complex would remain intact in the presence of sufficient organic solvent. In the absence of oxygen the mononuclear and the dinuclear complexes will reversibly interconvert, whereas, in the presence of oxygen, the complexes will oxidize. These studies document the existence of higher oligomeric vanadium compounds and surprisingly, in general, lend credibility to several emerging mechanistic proposals involving oligomeric species of vanadium compounds in catalytic processes.