Oxidation of Pu(III) in tri-n-butyl phosphate solutions containing nitric acid has been studied by ultraviolet/visible spectrophotometry. The concentrations of Pu(IV) and nitrous acid (HNO2), one of the reaction intermediates/products, were monitored. An autocatalytic reaction mechanism is proposed in which a small amount of HNO2 is generated during an induction period. It then catalyzes the reaction by reacting with nitric acid to produce NO2, which oxidizes Pu(III) in the rate-determining step. It is suggested a nitritoplutonium(IV) complex is formed that finally reacts with nitric acid to form more HNO2. Reaction rates, measured under conditions similar to those encountered in a solvent extraction contactor, indicate that the reaction proceeds rapidly at concentrations of HNO3 greater than ∼0.2 M in the organic solutions and it is responsible for plutonium losses often observed in solvent extraction operations.