A flow injection (FI) system with on-line liquid scintillation detection has been used to investigate and automate the separation of americium and plutonium on a sorbent extraction column (Eichrom TRU-resin). The FI system delivers various reagent solutions under controlled conditions to perform reactions that control the speciation of Pu, adjusting it between the III and IV states on the column. Am(III) is eluted in hydrochloric acid, while Pu(IV) remains on the column. The Pu is then selectively eluted by an on-column reduction reaction to Pu(III), which is not retained in hydrochloric acid. Remaining tetra- and hexavalent actinides are removed from the column using complexing eluent. Studies of the on-column oxidation of Pu with nitrite revealed that nitrite is slightly retained and partially converted to other species, both processes influencing subsequent reduction reactions. Mild reducing agents such as hydroquinone effected a slow and incomplete reduction of Pu(IV) on the column; examination of several other reducing solutions revealed that Ti(III) solutions gave rapid quantitative on-column reductions for selective Pu elution. The effects of column size and eluent flow rates were examined:  flow rates as high as 3 mL/min could be used without significant change in elution peak volume.