Spectrophotometric studies of the reaction of osmium with tin(II) chloride have been carried out. The optimum conditions [hydrochloric acid and tin(II) chloride concentrations and the time of heating] under which a stable, anionic complex of osmium, OsCl₂(SnCl₃)₂^2–(λ= 385 nm, ε= 2.4 × 10^–3 l mol^–1 cm^–1), is formed have been established. A sensitive method for the determination of microgram amounts of osmium, based on an ion associate with Rhodamine B (RB), has been developed. The ion associate is separated by flotation from 0.5 M HCl-2.5 × 10^–4M RB-0.1 M SnCl₂ with cyclohexane and dissolved in acetone, and its molar absorptivity is 4.1 × 10⁵ l mol^–1 cm^–1 at 560 nm. The solutions of the complex obey Beer's law in the range 0.05–0.4 µg ml^–1 of osmium. The relative standard deviation is 2–8%. The Os : RB molar ratio in the complex is 1 : 4, and its composition as the adduct [(RB+)₂OsCl₂(SnCl₃)₂^2–].2(RB+SnCl₃^–) has been established. Ruthenium reacts similarly. The determination of osmium is selective after its separation as OsO₄ from nitric acid. The method has been applied to the determination of osmium in mixtures of platinum metals.