Reaction between [VO(acac)₂] and H₂L (H₂L are the hydrazones H₂sal-nah I or H₂sal-fah II; sal = salicylaldehyde, nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) in methanol leads to the formation of oxovanadium(IV) complexes [VOL·H₂O] (H₂L = I: 1, H₂L = II: 4). Aerial oxidation of the methanolic solutions of 1 and 4 yields the dinuclear oxo-bridged monooxovanadium(V) complexes [{VOL}₂µ-O] (H₂L = I: 2, H₂L = II: 5). These dinuclear complexes slowly convert, in excess methanol, to [VO(OMe)(MeOH)L] (H₂L = I: 9, H₂L = II: 10), the crystal and molecular structures of which have been determined, confirming the ONO binding mode of the dianionic ligands in their enolate form. Reaction of aqueous K[VO₃] with the ligands at pH ca. 7.5 results in the formation of [K(H₂O)][VO₂L] (H₂L = I: 3, H₂L = II: 6). Treatment of 3 and 6 with H₂O₂ yields (unstable) oxoperoxovanadium(V) complexes K[VO(O₂)L], the formation of which has been monitored spectrophotometrically. Acidification of methanolic solutions of 3 and 6 with HCl affords oxohydroxo complexes, while the neutral complexes [VO₂(Hsal-nah)] 7 and [VO₂(Hsal-fah)] 8 were isolated on treatment of aqueous solutions of 3 and 6 with HClO₄. These complexes slowly transform into 9 and 10 in methanol, as confirmed by ¹H, ¹³C and ⁵¹V NMR. The anionic complexes 3 and 6 catalyse the oxidative bromination of salicylaldehyde in water in the presence of H₂O₂/KBr to 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde, a reaction similar to that exhibited by vanadate-dependent haloperoxidases. They are also catalytically active for the oxidation of benzene to phenol and phenol to catechol and p-hydroquinone.