Plutonium is thought to exist mostly as low soluble plutonium(IV) species in the environment and, therefore, has low potential of becoming mobile. Due to their prevalence and high solution stability constants for Pu(IV), microbial siderophores could significantly affect plutonium solubility and mobility. In this study, the ability of trihydroxamate desferrioxamine siderophores to solubilize Pu(IV) solids was investigated. Both desferrioxamine B and E (DFB and DFE) are far less effective at solubilizing amorphous Pu(IV) hydroxide in neutral solution than are simple chelators, such as EDTA, citrate, and tiron, despite the fact that these chelators have smaller solution stability constants for Pu(IV) than do DFE and DFB. Positively charged, linear DFB is less effective than the neutral cyclic DFE. Hydroxamate siderophores may, in fact, passivate Pu(IV) hydroxide surfaces, thereby inhibiting solubilization by other chelators. PuO2 solubilization under these conditions is far slower than that of Pu(IV) hydroxide.