Abstract: By studying spectrophotometrically the reaction kinetics of acetaldoxime and HNO₂ in both HNO₃ and HClO₄ medium, the corresponding equation of the reaction kinetics was obtained. It is found that the rate constant of the reaction of acetaldoxime and HNO₂ is (921.6±7.4) (mol/L)^－1.5·min^－1 when the temperatures is taken to be 20 ℃, correspondingly. It indicates that acetaldoxime is able to react with HNO₂ very quickly. Also, it is demonstrated that the velocity of the reaction speeds up with the increase of the acetaldoxime concentration, the HNO₂ concentration, and the temperature. Therefore, acetaldoxime can be used as the reductant for the separation of U and Pu in the Purex process of spent fuel without adding any other stabiliser, since it can scavenge HNO₂ completely by itself.