DFT including nonlocal corrections and ab initio calculations at MP2 and MP4 levels of theory have been performed in order to provide information concerning the mechanism of the rate limiting step of the thermal decomposition of methyl azide and ethyl azide. The chemically interesting points of the ground-state potential energy surface have been fully optimized, and a detailed normal-mode analysis for the reagents and the transition states is presented. The well-established scaled quantum mechanical force field method has been used to obtain reliable vibrational frequencies for these molecular structures. The force fields of transition states have been modified by using the scale factors computed for the force fields of the azides in their ground state. Finally, the activation energies and the Arrhenius preexponential factors for the rate constant have been computed according to transition state theory. The best values for the activation energies are provided by B3-LYP/6-311+G**. For the preexponential factor, the agreement with experiment seems to be independent of the level of theory used.