Reactions of phenyl azide with aromatic compounds (i.e. benzene, toluene, ethylbenzene, cumene, anisole, and bromobenzene) in the presence of AlCl₃ gave diarylamines, whereas those of phenyl azides with an electron-donating group such as Me and OMe yield little of the corresponding diarylamines (tar formation takes place). The reaction of 4-nitrophenyl azide with benzene in the presence of AlCl₃ gave 4-nitrophenyl(phenyl)amine and a C-substitution product, 2-amino-5-nitrobiphenyl. Decomposition of 2-azidophenyl(phenyl)methane and 2-azidobiphenyls in CH₂Cl₂ in the presence of AlCl₃ underwent an N-cyclization to afford 9,10-dihydroacridine and carbazoles, respectively, in good yields. The kinetic data on the decomposition of aryl azides and a Hammett plot (with ρ–6.0) for the formation of diarylamines suggest that the products are formed by an aromatic N-substitution of arylnitrenium–AlCl₃ complexes via azide–AlCl₃ complexes. We further argue the character of arylnitrenium–AlCl₃ complexes.