The oxidation of arsenious acid by Mn(Ⅲ ) which is produced from the reaction of potassium permanganate and arsenious acid can be catalyzed by iodide in the medium of sulphuric acid and phosphoric acid.Based on the catalytic role of iodide,a catalytic spectrophotometric method for the determination of trace iodide has been established with a detection limit of 0.4 μ g/L,linear range of 1.0~ 25.0 g/L,RSD of 2.5% ~ 3.7% (n=6),and recovery rate of 96.4% ~ 102.6% .The method has been satisfactorily us...