This study uses a combination of laser flash photolysis (LFP) and product analysis to show that singlet nitrenes from the irradiation of phenyl, 4-biphenylyl, and 2-fluorenyl azide can be trapped by protonation in aqueous solutions forming nitrenium ions. With phenyl azide, the phenylnitrenium ion is indicated by the formation of ring-substituted anilines in yields of up to 50% in 1 M acids. The acidity dependence furnishes the ratio k(H):k(exp) = 1.1, where k(H) refers to H+-trapping of singlet phenylnitrene and k(exp) to ring expansion of this species. With k(H) expected to be 2-4 x 10(10) M(-1) s(-1), k(exp) is therefore estimated as 2-4 x 10(10) s(-1). Protonation by solvent water also occurs, but even though the rate constant is of the order of 10(9) s(-1), it constitutes a minor pathway in competition with the ring expansion. LFP studies in acids reveal a transient that is assigned the structure of N-protonated 4-hydroxy-2,5-cyclohexadienone imine, the intermediate formed by water addition to the para position of the phenylnitrenium ion. With 4-biphenylyl- and 2-fluorenylnitrene, ring expansion (and intersystem crossing) occurs more slowly and protonation by water is faster, with the consequence that there are substantial yields of nitrenium ion without added acids. These nitrenium ions are detected with ns LFP, and their formation from singlet nitrene is observed with ps LFP. Combining the LFP experiments with product analysis furnishes a pK(a) value of 16 for the 4-biphenylylnitrenium ion deprotonating to singlet nitrene in 20% acetonitrile. Thus singlet 4-biphenylylnitrene falls close to the category of a strong base in this solution. LFP experiments in acids show behavior consistent with N-protonation of the nitrenium ion forming an aniline dication. Kinetic analyses furnish pK(a) values of 0.1 (4-aminobiphenyl dication) and 0.6 (2-aminofluorene dication) in 20% acetonitrile with 1 M ionic strength. This and other pieces of evidence are consistent with these arylnitrenium ions being better regarded as 6-iminocyclohexadienyl carbocations. Overall, arylnitrenium ions (ArNH+) are very weak acids in water in their deprotonation to singlet nitrenes. They are also weak bases, accepting a proton to form the aniline dication-(ArN)-Ar-1 reversible arrow (ArNH+)-Ar-1 reversible arrow (ArNH2)(2+).