Abstract: A catalytic spectrophotometric method based on the catalytic effect of Os(Ⅳ)on the oxidation of chlorophosphonazo-pC with KIO₄ in acidic medium has been proposed for the determination of trace osmium. The reaction was carried out directly in aqueous solution. The detection limit is 0.000 2 μg/mL and the Beer's law is obeyed in the range of 0.000 6～0.003 μg/mL. The method proposed has been applied to the determination of Osmium in noble metals ores with satisfactory results.