ABSTRACTSimple Hydroxamic acids (XHAs) are salt free, organic compounds with affinities for hard cations such as Fe³⁺, Np⁴⁺, Pu⁴⁺ and have been identified as suitable reagents for the control of Pu and Np in advanced nuclear fuel reprocessing. Building upon previous work on the neptunium(IV)-formohydroxamic(FHA) acid system [1], a model that describes the hydrolysis of the acetohydroxamate moiety has been extended to include hydrolysis of bishydroxamatoneptunium(IV) complex. The model has been used to determine the rate constants for hydrolysis of mono- and bis-acetohydroxamatoneptunium(IV) at 25 °C, which were found to be 1.0×10^-5 dm³ mol^-1 s^-1 and 5.0×10^-5 dm³ mol^-1 s^-1, respectively.