The nature of the two isomers of 4-nitroazoxybenzenes in strongly acidic solution have been analyzed by U.V. spectrophotometry. Oxygen atom in the azoxy-group of $alpha$-and $eta$-4-nitroazoxybenzene in strong acid solution migrates neither to para position nor ortho position of the unsubstituted benzene ring of the compound in contrast with the cases of methyl-and bromo-substituted azoxybenzene, and shown no Wallach rearrangement. Since the $^1A{ ightarrow}^1H$ bands of the spectra shown hyperchromic effect whereas the ${pi}{ ightarrow}{pi}^*$ bands of them exhibit extream hypochromic effect, it appears most likely that trans ${ ightarrow}$cis isomerization take place in the media. A mechanism of a triangular transition state by which the transformation might be proceeded, is proposed.