The ${alpha}-and;{eta}$-isomer of 4-nitroazoxybenzenes have been separated by liquid chromatography and their U.V. spectra were examined. The n${ o}{pi}^{ast}$ transition band of the compounds did not appear, likewise the cases of other compounds of the series. Transition band of the new isomer were as usual as those of other azoxy-compounds, whereas the ${pi}{ o}{pi}^{ast}$ transition band of the other isomer which is reported in the literature shown peculier hypochromic shift and hypochromic effect. From the spectroscopic point of view it is very likely that the new isomer (m.p. $184-5^{circ}C$) is ${alpha}$-isomer and the other one (m.p. $152^{circ}C$) is ${eta}$-isomer contrary to the literature.