Thiourea dioxide, readily obtained by the treatment of thiourea with hydrogen peroxide, has been proved to be a useful reducing agent for the preparation of some kinds of substituted aromatic amino compounds from the corresponding nitro compounds. Reduction of nitrobenzene (I) to aniline (IV) in an alkaline solution, showed that hydrazobenzene (VII) was formed from nitroso- (II), azo- (VI), and azoxy-benzene (V) in a good yield. Some substituted hydrazobenzenes were formed from the corresponding azobenzenes.