Controlled potential coulometric studies are carried out for the determination of plutonium in mixed (U, Pu) carbide fuel samples. Variations in coulometric blank and interference due to iron and organic impurities are investigated. These impurities are likely to come from the sample, reagent acids, and during dissolution of carbide samples. A method for the determination of plutonium involving the successive addition of sample solution aliquots directly into the coulometric cell is evolved and demonstrated. Employing this method, eight to ten aliquots, each containing 2 to 5 mg of plutonium, can be analyzed in the same electrolyte (25 ml 1 N H2SO4), thus gaining an appreciable reduction in the analysis time. Also, the volume of analytical waste is considerably reduced. Precision and accuracy within 0.2% are achieved in the routine analysis of fuel samples.