A Kinetic-spectrophotometric method for the determination of ultra-trace amounts of osmium(VIII) is described. It is based on the catalytic action of osmium(VIII) on the oxidation of pyrogallol red with hydrogen peroxide, yielding a colorless product in neutral medium. The reaction is followed spectrophotometrically by measuring the rate of change in absorbance at 540 nm and 30°C. Os(VIII) in the range 0.005 -100 ng.ml−1can be determined. The proposed method is hardly subject to interferences. The relative standard deviation is 1.5% for 1 ng.ml−1of Os(VIII). The kinetic parameters of the catalyzed and uncatalyzed reactions are reported. The detection limit is 1 pg.ml−1.