Abstract A method is described for the determination of plutonium in the range 4.10 −6 to 4.10 −4 M by a.c. pplarography.It is based on. a peak at −0.42 V vs. a mercury pool anode from a citrate supporting electrolyte (pH 4.5). This peak is produced by plutonium in either the trivalent or the tetravalent state, due to the reversibility of the Pu 4+/Pu 3+ coupler. Spectrophotometry has shown that the tetravalent plutonium-citrate complex is more stable than the trivalent complex, which oxidizes to the tetravalent state. The peak for tetravalent plutonium is preferred for analytical work, but unfortunately interference can be caused by some elements, especially iron. An ion-exchange procedure has been developed for the separation of plutonium from interfering elements. This method has been applied to the determination of the plutonium contents of uranium-plutonium alloys containing 0.25–1.0% of plutonium.