Much attention has been paid to the fact that quite a few herbicides such as phenylcarbamates, phenylureas, and acylanilides form azo compounds known as carcinogens by virtue of the microoranisms in soil. In consequence, many investigators synthesized. TCAB, an azo compound, starting from 3,4-dichloronitrobenzene for the related studies. However, the authors were under the necessity of synthesizing ^(14)C-ring-labeled TCAB from ^(14)C-ring-labeled 3,4-DCA available, in addition to making up for the disadvantage of dechlorination in the reduction of 3,4-dichloronitrobenzene. The new method is as follows:TCAB, ^(14)C-ring-labeled and non-labeled, was produced by aerial oxidation of 3,4-DCA catalyzed by CuCl with pyridine as solvent at 60℃ for 5-12 hrs, giving 80.2% yield. The procedure forpurification was described in detail. The identities of TCAB isomers were confirmed by means of autoradiography, TLC, GLC, IR, and MS.