Irradiation of 2-azidofluorene in an aqueous solution containing 2 '-deoxyguanosine (dG) gives good yields of 8-(2-fluorenylamino)dG. This is the C8 adduct implicated in the carcinogenicity of 2-aminofluorene, and formed in vivo from the reaction of DNA-guanine and an ester of N-hydroxy-2-aminofluorene. Flash photolysis of the azide reveals two intermediates on the pathway that forms this adduct, the 2-fluorenylnitrenium ion, and a subsequent longer-lived species formed in the reaction of this ion with de. A number of pieces of evidence identify this later intermediate as the initial Cs adduct derived from addition of the nitrenium ion to the C8 carbon prior to loss of the C8 proton. Both spectroscopic and kinetic analyses show that the latter actually exists in both a cationic acid form and a base form, with a pK(a) for the acid of 3.9. The base form is a tautomer of the final product and is the species present at pH 7. There is also evidence that the reaction of the nitrenium ion and dG that forms this intermediate proceeds directly by addition at C8. In other words, the substitution of ArNH+ for H+ at the C8 position of 2 '-deoxyguanosine is a straightforward electrophilic aromatic substitution.