A series of para-substituted N-methyl-N-phenylnitrenium ions (N-(4-biphenylyl)-N-methylnitrenium ion, N-(4-chlorophenyl)-N-methylnitrenium ion, N-(4-methoxyphenyl)-N-methylnitrenium ion, and N-(4methylphenyl)-N-methylnitrenium ion) were generated through photolysis of the appropriately substituted 1-aminopyridinium salt. Laser flash photolysis using UV-vis detection as well as photoproduct analysis verified that the expected nitrenium ions were formed cleanly and rapidly following photolysis. Laser flash photolysis with time-resolved infrared detection allowed for structural characterization of the nitrenium ions through observation of a symmetrical aromatic C=C stretch in the region 1580-1628 cm(-1). The specific frequencies reflect the degree of quinoidal character present in each phenylnitrenium ion (i.e., the degree to which the nitrenium ion resembles a 4-iminocyclohexa-2,5-dienyl cation). The 4-methoxy derivative shows the highest frequency C=C stretch, indicating that this strongly pi-electron-donating substituent imparts more quinoidal character, and the 4-chloro derivative shows the lowest frequency C=C stretch, suggesting that it possesses the least quinoidal character. Quantum calculations using density functional theory (BPW91/cc-pVDZ) were carried out on the same nitrenium ions. The theoretically derived IR frequencies showed excellent quantitative agreement with the experiment. The computed structures show significant bond length alternation in the phenyl rings, shortened C-N bond lengths, and substantial positive charge delocalization into the phenyl rings. All of these effects are more pronounced with increasing pi-donating character of the ring substituent. Arylnitrenium ions are well described as 4-iminocyclohexa-2,5-dienyl cations.