ESR spectra of iminoxy radicals chemically produced in 1-nitroso 2-naphthol, 1-nitroso 2-naphthol 3, 6-disulfonic acid, 2-nitroso 1-naphthol, 2-nitroso 5-dimethylaminoaphenol, and 4-nitroso phenol were observed. These radicals were assigned to be produced via less stable nitroso naphtho or nitroso benzo semiquinone radicals, which were first derived in nitroso compounds by oxidations. Two isomers, Z type having nitrogen hyperfine coupling constants of ca. 2.7 mT and less stable E type with those of ca 3.1 mT were involved in 1-nitroso naphthol, 1-nitroso 2-naphthol 3.6-disulfonic acid, and 2-nitroso 5-dimethyl amino phenol while the E type in 2-nitroso 1-naphthol was too weak to be detected. The only one iminoxy radical was detected in 4-nitroso phenol. On UV irradiations of 1-nitroso 2-naphthol at -100℃, the less stable E type was detected in almost same intensity as the Z type. The structures and stability of the iminoxy radicals were concluded to be affected by the carbonyl group adjacent to the iminoxy one.