The effect of acetohydroxamic acid (AHA) on speciation of transuranic elements has been investigated in the extraction and spectrophotometric experiments relevant to UREX extraction process. The distribution ratios decreased with increased concentration of added AHA rapidly, and a considerable ability of AHA to form complexes with plutonium even under strong acidic conditions was confirmed. Using FITEQL 4.0, a computer modeling program for equilibrium experimental data, the speciation distribution of tetravalent plutonium was determined. Acetohydroxamate species of Pu(IV) identified in Vis-NIR spectra of the extraction organic phase confirms formation of extractable neutral solvates of ternary acetohydroxamate-nitrate complexes of plutonium with tributyl phosphate (TBP).