Presidency College, Calcutta-700 073, India Manuscript received 20 July 1979. accepted 30 December 1979 N-α-pyridy-N'-benzoyl thiourea (PBT) is recommended as the most suitable reagent for either gravimetric or spectrophotometric determination of iridium. The precipitate formed by the reagent with iridium(III) at pH 4.2-5.86 can be weighed as Ir (C13H10N3OS)3. The base metals do not interfere with the gravimetric determination of iridium in presence of masking agents. Interferences of platinum (IV) and palladium(III) are avoided by prior precipitation of the metal ions with PBT in 1.5(N) hydrochloric acid medium. Ruthenium(III) and osimium(VI) when reduced with hydroxyl-amine hydrochloride and sulphur dioxide respectively, produce no interference in the gravimetric determination of iridium. The iridium-PBT complex can be extracted with chloroform at pH 5.2-6.4 for the spectrophotometric determination of iridium. The extract shows maximum absorbance at 350 nm, obeys Beer's law at the same wavelength over the concentration range of the metal from 0.816 μg/ml to 8.16 μg/ml . By the use of PBT, iridium can be spectrophometrically determined in the presence of large amounts of all the base metals and noble metals except rhodium.