A new procedure for the determination of uranium by packed-column supercritical chromatography is proposed here. 1-Phenyl-3-methyl-4-benzoylpyrazolin-5-one (HPMBP) is chosen, as it can easily extract uranium from a nitric acid aqueous phase in the presence of a synergistic agent. Eighteen columns packed with C18-grafted silica have been tested. Adding the chelating agent directly to the mobile phase greatly improves the elution peak shapes. Trioctylphophine oxide (TOPO), diphenyl sulfoxide, and phenanthrene have been studied as synergistic agents. The influence of the injection solvent is also shown, and a calibration curve is obtained for uranium in the range of 20 ng to 2 mug injected. The limit of detection is 2 mug of uranium. The accuracy of the method is 4%, and the repeatability measured (1 sigma) is 3%.