Interaction of solutions of uranyl formate or acetate with mycobactin S in anhydrous organic solvents leads to the formation of 11 complexes which can be isolated as dark orange, vitreous, non-hygroscopic solids. Aqueous extraction measurements with the232U-labelled complexes show that extensive dissociation occurs in acidic medium. On the other hand, the compounds are highly resistant to photolytic decomposition. Spectrophotometric characterization and structural considerations suggest that coordination geometry of the ionophore with UO
is practically identical in both derivatives, and may impose sterical requirements different from those of other metalmycobactin complexes.