Kinetics of the oxidation of phosphorous acid by vanadium(V) over a temperature range of 40-55 °C have been studied. The reaction is first order with respect to vanadium(V) and phosphorous acid. The reaction is acid catalysed and is faster in sulphuric than perchloric acid. The order of the reaction with respect to mineral acid, when [Acid] ≤ 5.5M, is two. The effect of dielectric constant on the rate indicates that the reaction is of the ion-dipole type. Activation parameters have been evaluated. The reaction is initiated by formation of free radicals in the slow rate-determining step.