Extraction of plutonium(IV) from aqueous sulfuric and sulfuric-nitric acid into di-2-ethylhexyl phosphoric acid (HY) in the diluents n-dodecane, toluene or chloroform has been investigated. The composition of the Pu(IV) species extracted from H2SO4 was found to be PuH2Y6, which changed to Pu(NO3)H2Y5 and Pu(NO3)2H2Y4 with increasing concentration of nitrate ion in the aqueous medium. These three species can be represented as PuY2(HY2)2, Pu(NO3)Y(HY2)2 and Pu(NO3)2(HY2)2, respectively, where Y represents the anion of monomeric HY, and HY2 the anion of dimeric H2Y2. Synergism in the extraction of Pu(IV) by the addition of thenoyltrifluoroacetone (HTTA) to HY was also investigated and attributed to extraction of the additional species, Pu(TTA)Y(HY2)2 and Pu(TTA)2(HY2)2. The addition of the neutral extractant tri-n-octylphosphine oxide (TOPO) to HY did not result in synergism in the extraction of Pu(IV) from aqueous sulfuric acid. With aqueous nitric acid under similar conditions, however, synergism was observed. The possible equilibria in these systems were identified and the corresponding equilibrium constants were determined.