Indirect potentiometric procedures with chloramine-T (CAT) as the oxidant and a chloramine-T ion-selective electrode are described for the determination of hydrazine and isonicotinic acid hydrazide (isoniazid) in the 1–100 μmole range and of sulphide and thiosulphate in the 0.5–50 μmole range. The reductants react stoichiometrically with a known and excessive volume of added CAT and the unconsumed excess of CAT is measured with the CAT-selective electrode. Aqueous solutions of the reductants were analysed with an error and precision of about 1–2%. Analytical recovery of isoniazid added to tablet diluents was 97–101% (average 98.9%). Results were comparable with those obtained with a titrimetric method for the determination of isoniazid in injection solutions and tablets.