WE have determined the kinetic isotope effect in the iodination of many aromatic amines, amino-carbonic and aminosulphonic acids by comparing the iodination-rates of normal and deutero-compounds. The results are collected in Table 1, the isotope effect being the ratio of iodination-rate constants, k H and k D of normal and deuterated compounds without any correction for percentage of substitution of H by D). Relative activity, A r is the ratio of the rate constant of the given compound to that of iodination of m-dimethylaminobenzenesulphonate ion taken as 1.