A Ssimple, rapid and accurate potentiometric method is described for the determination of Os(VIII) in the concentration range 0.4 – 4.0 mg/ml. the method is based on the addition of hydrazine hydrochloride to Os(VIII) to reduce it to Os(IV). The excess of hydrazine hydrochloride was oxidized tby iodine dissolved in acetic acid. The librated iodide was then potontiometrically titrated against mercury(II) using silver amalgam as the indicator electrode. The potential of this method for the sime- microdetermination is important in practical applications where the above reaction should proceed quantitatively towards completion. The relative standard deviation for six replicate determination of osmium(VIII) in binary and ternary mixtures without the need for extraction or heating. Sci. Pharm., 69, 367–374 (2001).