Abstract: A micro--iodine ion selective electrode is developed to follow and monitor thecourse of oxidation of iodide ion during radioactive iodination operation. Ourexperimental results indicate that this method can respond quickly to the courseof oxidation if the reacting liquid is greater than 30 μl in volume. Therefore itcan be used for accurately controlling the amount of oxidizing reagent used for thereaction, for example, the amount of chloroamine T can be reduced to 1/40 ofthe amount ordinarily used for the preparation of Angiotonin Ⅱ and insulin.The effect of pH and concentration of phosphate of the reacting liquid to theoxidation reaction of Ⅰ with chloroamine T is also studied.