4-Azido-X'-substituted biphenyls (X' = 4'-MeO, 4'-Me, 4'-F, 3'-Me, 4'-Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions10(Ar C6H4-N+H) are observed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). These cations are quenched by azide ion, with values ofkazranging from 6 to 10 x 109M-1s-1, with the majority in the range (9-10) x 109. This near constantkazprovides further evidence that arylnitrenium ions are quenched by azide ion at the diffusion limit. The solvent reactivities, plotted in a single-parameter Hammett plot against &sgr; sup + (X), exhibit a poor correlation, with the points for thepara&pgr;-electron donors deviating from the correlation line based on the other substituents in the direction of requiring a more negative substituent parameter. The data are more satisfactorily fit to the two-parameter Yukawa-Tsuno equation; the parameterr+obtained in this fit is 2.8. Thus the resonance interaction of thepara&pgr; -donor X'-substituents with the positive charge of the cation is underestimated by &sgr; sup + , a situation that has previously been observed with benzylic-type carbenium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two additional stabilizing vinyl groups, i.e., as if they had the structure Ar-C+(C = C)2. The inherent reactivity and the pattern of the aryl substituent effects are in fact similar to those in the carbocation series Ar-C+(Ph)2.Key words: nitrenium, nitrene, aryl azide, photochemistry, lifetime.