The optical density of KNO 3 crystals was increased from 240 through 1000 mμ by exposure to x or γ-irradiation at 77 or 4.2°K. The increase is accounted for by the growth of seven radiation-induced absorption bands, termed, I, II, III, IV, V, VI and VII. The response of the individual bands to selective optical bleaching was determined. Light which reduced optical absorption bands was also observed to selectively reduce signals in the electron paramagnetic resonance (EPR) spectra of similarly-irradiated KNO 3 crystals. These EPR signals had previously been assigned to specific nitrogen-oxygen radicals. A tentative identification of bands I and V with NO −2 2 band VI with NO 3, band VII with NO 2, and band III with NO 2− 3, was made on the basis of correlalations between the EPR and optical response. The known extinction coefficients of NO 2 −, NO 2 etc., together with their measured concentrations, were also considered in making this identification scheme. The response of the optical bands to the selective optical bleaching is accounted for on this scheme by electron release from NO 2− 2 and capture by NO 3 and NOa, hole release from NO 2, and photolysis of NO 2− 3 by 325 mμ light. Molecular orbital descriptions of the species are discussed to deduce approximate positions, E max, and oscillator strengths, f;, of transitions in the proposed species. The deduced values of E max are in qualitative agreement with the observed positions of maximum absorbance at 510, 355, 325, 290, 270, 246 and 410 mμ for bands I–VII respectively.