The photochemical decomposition of nitric oxide has been studied over a pressure range 60 to 500 mm., using a low pressure mercury arc source and a circulating system. Stoichiometrically, the reaction was found to be represented by 6NO → N2 + 2N2O3. Addition of nitrogen decreased the rate, as did also a decrease in the mercury vapor pressure. Using appropriate light filters at 1.7 × 10−3 mm. mercury pressure the decomposition was shown to be photosensitized by Hg (61P1) atoms. The primary process is probably dissociation of nitric oxide into nitrogen and oxygen atoms.