The reaction chemistry of the potent insulin-mimetic agent bis(maltolato)oxovanadium(IV) (abbreviated BMOV or VO(ma)(2)) is reported. VO(ma)(2) (log K-1 = 8.80(2), log K-2 = 7.51(2), log beta(2) = 16.31(3)) has a rich coordination chemistry, forming a number of V(IV) and V(V) derivatives. In aqueous solution it is slowly oxidized by molecular oxygen to [VO2(ma)(2)](-) (log K-1 = 7.5(1), log K-2 = 6.2(1), log beta(2) = 13.7(1)); in alcohols a variety of V(V) analogs VO(OR)(ma)(2) (R = CH3, C2H5, i-C3H7) are formed by aerial oxidation. All these vanadate complexes can be interconverted by reaction with the appropriate alcohol or water. In addition, the six-coordinate V(IV) pyridine adduct VO(ma)(2)py can be formed and this undergoes oxidation to V(V) complexes much more slowly, demonstrating that a vacant coordinate site is required for the coordination of O-2 to VO(ma)(2) before inner-sphere oxidation can take place.