Abstract The reaction of peroxodiphosphate with vanadyl ion in dilute aqueous acid has been investigated. In the absence of other reducing agents, two vanadyl ions are oxidized by one peroxodiphosphate. The kinetics are first order each in oxidant and reductant. The rate is not markedly dependent on pH in the range from 1.1 to 3.5. The significant change in stoichiometry when organic buffers were employed indicates the presence of a reactive intermediate. The interaction of vanadyl ion with other species such as vanadium(V) and pyrophosphate to form complexes is seen from the variation of absorbance with concentration of additive. Because of these complications, the rate plots show curvature and there is a larger error than usual in the rate constants. Nevertheless, the results lead to reasonable conclusions concerning the reaction mechanism.