The solution structure of the complex produced by the condensation of 2 mol of vanadate with 2 mol of a vicinal diol in aqueous solution has been characterized. This characterization is based on an assessment of the effect of water on the equilibrium constant for formation of the dimeric ethylene glycol-vanadate complex at high and intermediate ethylene glycol concentrations, on V-51, C-13, H-1, and O-17 NMR studies, and on an evaluation of the vibrational frequency of the VO stretching mode for non-ester oxygens in the natural abundanceand O-18-labeled dimers, using Raman difference spectroscopy and Fourier transform infrared difference spectroscopy. The solution structure is analogous to that previously reported in the solid state for the related dimeric vanadium complexes of 2-ethyl-2-hydroxybutyric acid and of pinacol. That structure contains a four-membered V2O2 unit within the framework of two VO5 clusters that each are part of a five-membered ring involving the diol. The vibrational properties of the VO5 cluster in the dimeric ethylene glycol-vanadate complex are compared with those in dimers produced by the condensation of vanadate with a-hydroxy acids and of VOCl3 with ethylene glycol and pinacol. H-1 and C-13 NMR studies are used to investigate the interconversion of isomeric forms of two dimeric 1,2-diol-vanadate complexes and one dimeric a-hydroxy acid-vanadate complex. Several conclusions can be drawn about the structures of five-coordinate vanadates and vanadate-based transition-state analog complexes with phosphotransferases on the basis of the proposed structure for these complexes.