Milligram amounts of uranium dioxide can be quantitatively dissolved in sup
ercritical carbon dioxide containing a complex of tri-n-butyl phosphate (TB
P) with nitric acid and separated from thorium(IV). The quantitative dissol
ution of milligram amounts of solid uranium trioxide in supercritical carbo
n dioxide containing thenoyltrifluoroacetone (TTA) and TBP was performed us
ing ultrasonication. The separation of uranium(VI) and cerium(IV) in the te
st system was demonstrated.