The controlled-potential coulometric determination of plutonium with a concurrent measurement of an interfering reversible couple is described. Sample electrolysis over two different potential spans is used. The Nernst equation is utilized to determine the fraction electrolyzed for the plutonium and the interfering reversible couple for each potential span. Two linear equations, in two variables, each containing the appropriate data from one of the sample electrolyses are solved simultaneously for the quantities of plutonium and interfering reversible couple present. Appropriate background current corrections are discussed. This concurrent measurement technique has been applied to the determination of plutonium in solutions containing either plutonium and iron or plutonium and neptunium with a reliability of better than 0.1% for plutonium.