A nitrosamine photooxidation reaction is shown to generate a peroxy intermediate by experimental physical-organic methods. The irradiation of phenyl and methyl-substituted nitrosamines in the presence of isotopically labeled 18-oxygen revealed that an O atom was trapped from a peroxy intermediate to trimethylphosphite or triphenylphosphine, or by nitrosamine itself, forming two moles of nitramine. The unstable peroxy intermediate can be trapped at low temperature in postphotolyzed solution in the dark. Chemiluminescence was also observed upon thermal decomposition of the peroxy intermediate, that is, when a postphotolysis low-temperature solution is brought up to room temperature. A DFT study provides tentative information for cyclic nitrogen peroxide species on the reaction surface.