The oxidation of U(IV) ions in the diluted solvent phase, 30% TBP/n-dodecane, has been investigated in the presence of plutonium ions, which can act as catalysts for U(IV) oxidation. The reaction was shown to follow the cycle below, with the first and third stages being rate determining.U⁴⁺+ 2Pu⁴⁺+ 2H₂O → UO₂²⁺+ 2Pu³⁺+4H⁺2Pu³⁺+ HNO₃+ 2H⁺→ 2Pu⁴⁺+ HNO₂+ H₂OPu³⁺+ HNO₂+ H⁺→ Pu⁴⁺+ NO + H₂O2NO + HNO₃+ H₂O ⇔ 3HNO₂The overall reaction stoichiometry is the same as for the oxidation of U(IV) by HNO₃in TBP:U(NO₃)₄·2TBP + HNO₃·TBP + H₂O·TBP + TBP ⇔ UO₂(NO₃)₂·2TBP + HNO₂·TBP + 2HNO₃·TBPThe rate equations of both these rate limiting steps have been determined, with that for the U(IV)-Pu(IV) reaction (5) being given by the equation below, wherek₁=74.4±6 M^-1.2min^-1at 25.2 °C and the activation energy is 72±11 kJ mol^-1(in 0.5 M HNO₃).The rate of the second slow stage, the Pu(III)-HNO₂reaction, is given by the equation below, where the rate constant isk₂=627±28 M^-1min^-1at 25.2 °C and the activation energy is 87.2±1.4 kJmol^-1(in 0.5 M HNO₃).Mechanistically, it was shown that the U(IV)-Pu(IV) reaction may proceedviathe interaction of the hydrolysed actinide ions U(OH)₂²⁺and PuOH³⁺and the Pu(III)-HNO₂reaction was found to most probably involve oxidation of Pu(III) ions by nitrinium nitrate (NONO₃) ions in its rate determining step.