A series of substituted 1,2-didehydroazepines was prepared by photolysis of their precursor aryl azides. The chemical and physical properties of the didehydroazepines were probed by means of conventional chemical trapping experiments and by time-resolved spectroscopy with IR and UV detection. The experimental results are compared with predictions from MNDO calculations. The didehydroazepines are formed from the azides with efficiencies that depend systematically on the nature of the substituent. In some cases, didehydroazepines are not formed at all. The didehydroazepines react relatively rapidly with starting aryl azide and with added amines. The rate of their reaction is largely controlled by the electronic properties of the substituent on the didehydroazepine. These results permit prediction of the reactivity of didehydroazepines from their structure.