Introduction of +R type para substituents into azidobenzenes causes very large increases in rate of thermolysis, up to 225-fold. The rates of nitrobenzene solutions at 120 °C follow a Hammett-type linear free energy relationship log k = -5.44 - 2.33Ï I - 1.48 R⁺ which indicates conjugative stabilization of a nitrene-like transition state. ortho-Substituents of the +R type causes still larger rate enhancements, up to 456-fold for 2-amino, which identify a special resonance proximity effect. It is suggested that the very high rates reported for such α-azidoheterocycles as 2-azidothiophene are due to similar resonance stabilizations and not to ring-opening concerted with nitrogen loss.