Kinetic measurements were carried out for the outer-sphere electron transfer reactions involving [VO(Schiff-Base)] +/0 couples. Electron self-exchange rate constant for [VO(3-MeOsal-(RR)-chxn)] +/0 couple was determined as k ex =(5.2±0.8)×10 6 kgmol −1 s −1 at 25°C. It was found that added water in acetonitrile solvent retarded the electron transfer reactions between [VO(salen)] + and [Co(o-phen) 3 ] 2+ : the six-coordinate V(V) species, [VO(salen)(OH 2 )] + was found to be ca. 3.5 times less reactive compared with the five-coordinate species, [VO(salen)] + . This small difference between the reactivity of five- and six-coordinate species indicates that the reaction through the direct OV(V)–OV(IV) interaction is outer-sphere in nature, contrary to the previously proposed inner-sphere mechanism. Activation volumes corresponding to the electron self-exchange reactions for [VO(salen)] + /[VO(salen)] 0 and [VO(salen)OH 2 ] + /[VO(salen)] 0 couples were estimated from the volume profiles of the reduction reactions of [VO(salen)] + by [Co(o-phen) 3 ] 2+ , with and without added water in acetonitrile: ΔV*=−3.4±3.7 and −8.0±4.0cm 3 mol −1 for [VO(salen)] + /[VO(salen)] 0 and [VO(salen)OH 2 ] + /[VO(salen)] 0 couples, respectively. It was suggested that previously reported chiral recognitions in the redox reactions involving [VO(Schiff-Base)] +/0 redox couples take place within the encounter complex through the outer-sphere mechanism by maximizing the direct coupling between the dπ orbitals of V(IV) and V(V) species (Scheme 2).