Abstract: The accurate determination of Pu content in plutonium sulfate tetrahydrate by controlled potential coulometry is described. The plutonium sulfate tetrahydrate is dissolved in 2.5 mol/l HNO_3. A solution of l mol/l nitric acid is used as the electrolyte and netted gold as the working electrode. The Pu (Ⅳ) is reduced to Pu (Ⅲ) by maintaining the control potential at 0. 455 V (SCE), and then oxidation of Pu (Ⅲ) to Pu (Ⅳ) is accomplished by maintaining the control potential at 0.895 V (SCE) until the cell current level dropped down to less 20 μA. The amount of plutonium to be assaied is about 6 mg and the method shows a precision of 0.046% relative standard deviation, RSD, with no significant bias.