Photolysis of arylazides with long wavelength ultraviolet (NUV) light in an aqueous medium produces short-lived reactive chemical species which bind to DNA and deoxynucleoside 3'-phosphates and induce reversion mutations in frameshift tester strains of Salmonella typhimurium. Nitrenes are known reactive products of azide photolysis, so the DNA-binding and mutagenic species is either a nitrene or a nitrene-derivative. An N-hydroxyarylamine intermediate, potentially formed from a nitrene and water, can be excluded because the mutagenic potencies of the reactive species in TA98 and in the hydroxylamine-resistant TA98/1,8-DNP6 are of the same order, and because the life-time of this species is very short. The mutagenic potency of the arylazide photolysis products decreases in the order azido-IQ greater than 1-azidopyrene greater than azido-MeIQx greater than 6-azidochrysene greater than 2-azidofluorene greater than 4-azidobiphenyl greater than 2-azido-naphthalene greater than 1-azido-naphthalene. This potency sequence correlates with that of the corresponding arylamines. Furthermore, their DNA binding products are chromatographically identical with those obtained in cellular, metabolizing systems from nitroarenes and arylamines. Therefore, the reactive, electrophilic azide photolysis product is very likely a nitrenium ion formed by protonation of a nitrene. Nitrenium ions are also the ultimate mutagens/carcinogens formed from nitroarenes and arylamines. Arylazides can therefore be considered as stabilized forms of arylnitrenium ions. The arylazide-nitrene technique reported here is new and simple and provides ready access to presumed nitrenium ions which are otherwise difficult to obtain.