Spectrophotometric methods were used to study the kinetics of Np(VI) reduction to Np(V) and Np(IV) in 30% TBP solution in n-dodecane. The first reaction is limited by the stage in which solvated TBP molecules of NpO2(NO3)2 interact with hydrolyzed forms of U(IV), i.e. U(OH2)2(NO3)2 ions. Accordingly, the reaction rate is catalyzed by water and inhibited by nitric acid. The reaction proceeds at a much slower rate in a TBP solution than in an aqueous solution. The rate of Np(V) reduction with uranium(IV) in a TBP solution is independent of [HNO3] or [H2O] and proceeds at a substantially higher rate than in an aqueous solution. At the limiting stage, the transport of charge takes place between the reagents in their basic forms, NpO2NO3 and U(NO3)4. The reaction is affected by the formation of the non-reactive complex [NpO2U(NO3)5].