Reaction of vanadyl sulfate with 3-hydroxy-3-methylglutarate yields a complex which was shown by X-ray crystallography to be a vanadium-oxo dimer. The structure has two 3-hydroxy-3-methylglutarate trianions acting as tridentate ligands, each bridging the vanadiums through the hydroxy oxygen and each bonding to the different vanadiums through carboxylate oxygens. A temperature dependent magnetism study on the crystalline complex shows that the two vanadium(IV) S = 1/2 centers are antiferromagnetically coupled with a coupling constant of J = -30 cm-1, mu(expt) (298K) = 0.6 mu(B)/vanadium. The V=O stretching frequency in the IR was assigned for the complex using oxygen exchange in O-18 labelled water : (V=O-16 appears at 976 cm-1). Crystal data : Na2[VO(OOCCH2CO(CH3)CH2COO)]2.3H2O.CH3OH, V2Na2O16C13H24, orthorhombic, space group = Pn21a, a = 18.032(3) angstrom, b = 10.071(3) angstrom, c = 12.899(3) angstrom, V = 2342.3(16) angstrom3, Z = 4, 1630 observed unique data, R = 5.07%, R(w) = 6.04%, all measurements at -130-degrees-C.