Abstract: The kinetics of isotopic exchange reaction in o-iodohippuric acid (or o-iodobenzoic acid) and the effects of copper sulfate, reaction temperature and reaction time for the labeling efficiency of o-iodohippuric acid (or c-iodobenzoic) are studied. The results show that copper sulfate has an important catalytic effect on the labeling of o-iodohippuric acid. It not only lowers the temperature of labeling o-iodohippuric acid, speeds up the reaction and gives high yield, but also can control the formation and labeling of o-iodobenzoic acid. The reaction mechanism of the catalytic effect of copper sulfate is also discussed.