A simple and highly sensitive spectrophotometric method has been described for the determination of trace amount of osmium(VIII) using ethylene thiourea (ETU) as a chromogenic reagent. The method is based on the formation of an instantaneous purple coloured complex at room temperature by the reaction of osmium(VIII) with ETU in strongly acidic (pH=1) solution having absorption maximum at 490 nm and is confirmed using derivative spectrophotometry. Linear calibration graphs are obtained for 0.03-3 mu g/mL of the analyte. Sandell's sensitivity (1.13x10(-3) mu gcm(-2)), molar absorptivity (16.87x10(4) Lmol(-1) cm(-1)), detection limit (0.0291 mu g/mL) and quantitation limit (0.0833 mu g/mL) are also calculated. The method is optimized and different analytical parameters were evaluated. The stoichiometry of the complex is found to be 1:3 by Job's method and mole ratio method. The stoichiometry of the complex is further confirmed by synthesizing the solid compound and characterizing it by various physicochemical methods.