Allylthiourea is proposed as a reagent for the spectrophotometric determination of osmium(VIII). Osmium(VIII) forms a 1 : 1 complex with allylthiourea. Conformity to Beer's law was observed for up to 20 µg ml–1 of osmium in acidic medium (molar absorptivity 2.17 × 104 l mol–1 cm–1, at 298 nm); the Sandell's sensitivity of the reaction was 0.0087 µg cm–2 per 0.001 absorbance unit. The effect of foreign ions was also studied. A statistical evaluation of the experimental results was undertaken and a comparison with the most important recent spectrophotometric reagents for osmium is presented. The proposed method has the advantages of simplicity, reasonable sensitivity and rapid determination without the need for extraction or heating.