Abstract: Studies on the potentiometric determination of Pu (Ⅲ) and Pu (Ⅳ) in nitricacid solution containing hydrazine are reported. The usual redox titration for thedetermination of Pu (Ⅲ) and Pu (Ⅳ) is modified in order to overcomethe interference of hydrazine. In the modified method, three aliquots of the samesample are needed. According to the titrating results of three aliquots, the quanti-ties of Pu (Ⅲ), Pu (Ⅳ) and hydrazine can be determined. The reliability andreproducibility of this method are examined and reporfed. The relative errors forthe assay of Pu (Ⅲ) and Pu (Ⅳ) are found less than 0.5% in the range of mil-ligram to a few tens milligram.