The oxo anions of technetium-99m and rhenium, pertechnetate (TcO4-) and perrhenate (ReO4-), have been separated by high performance liquid chromatography on porous graphitic carbon with aqueous trifluoroacetic acid or salt solutions as eluent. Chromatographic retention was a result of electronic interaction between the lone pair electrons of the anions and the delocalised pi-electron clouds of the porous graphitic carbon. Retention and resolution can be controlled by the concentration and/or species of eluents which can compete with the solutes for electronic interaction being used for elution.