The vanadium(IV) compound [(VO)-O-IV(OH)((t)Bu(2)bpy)(2)]BF4 ((VO)-O-IV(OH)) ((t)Bu(2)bpy = 4,4'-di-tert-butylbipyridine) is slowly oxidized by O-2 in ethereal solvents to give the oxo-peroxo compound [(VO)-O-V(O-2)((t)Bu(2)bpy)(2)]BF4 ((VO)-O-V(O-2)) in excellent yield. This and related compounds were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an X-ray crystal structure of the 4,4'-dimethylbipyridine analog, [(VO)-O-V(O-2)(Me(2)bpy)(2)]BF4. Monitoring the reaction of (VO)-O-IV(OH) with O-2 in THF/acetonitrile mixtures by H-1 NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compound [V-V(O)(2)((t)Bu(2)bpy)(2)]BF4 ((VO2)-O-V), which then converts to (VO)-O-V(O-2). Reaction of (VO)-O-IV(OH) with O-18(2) gives ca. 60% triply O-18 labeled (VO)-O-V(O-2). The mechanism of formation of (VO)-O-V(O-2) is complex and may occur via initial reduction of O-2 at vanadium(IV) to give a superoxo-vanadium(V) intermediate, autoxidation of the THF solvent, or both. That (VO2)-O-IV is generated first appears to be due to the ability of (VO)-O-IV(OH) to act as a hydrogen atom donor. For instance, (VO)-O-IV(OH) reacts with (VO)-O-V(O-2) to give (VO2)-O-V. (VO)-O-V(O-2) is also slowly reduced to (VO)-O-IV(OH) by the organic hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by triphenylphosphine. Notably, the peroxo complex (VO)-O-V(O-2) is much less reactive with these substrates than the analogous dioxo compound (VO2)-O-V.