The kinetics of oxidation of bis(maltolato)oxovanadium(IV), BMOV or VO(ma)(2), by dioxygen have been studied by UV-vis spectroscopy in both MeOH and H2O media. The VO(ma)(2):O-2 stoichiometry was 4:1. In aqueous solution, the pH-dependeut rate of the VO(ma)(2)/O-2 reaction to generate cis-[VO2(ma)(2)](-) is attributed to the deprotonation of coordinated H2O, the deprotonated species [VO(ma)(2)(OH)](-) being more easily oxidized (k(OH) = 0.39 M(-1) s(-1), 25 degrees C) than the neutral form VO(ma)(2)(H2O) (k(H2O) = 0.08 M(-1) s(-1), 25 degrees C). The activation parameters for the two second-order reactions in aqueous solution were deduced from variable temperature kinetic measurements. In MeOH, VO(ma)(2) was oxidized by dioxygen to cis-VO(OMe)(ma)(2), whose structure was characterized by single-crystal X-ray diffraction; the crystals were monoclinic, C2/c, with a = 28.103(1) Angstrom, b = 7.721(2) Angstrom, c = 13.443(2) Angstrom, beta = 94.290(7)degrees, and Z = 8. The structure was solved by Patterson methods and was refined by full-matrix least-squares procedures to R = 0.043 for 1855 reflections with I greater than or equal to 3 sigma(I). The kinetic results are consistent with a mechanism involving an attack of O-2 at the V(IV) center, followed by the formation of radicals and H2O2 as transient intermediates.