Thermal decomposition process in the ground singlet state of several five-membered alpha-azido heterocycles analogous of azido-pyrazoles producing molecular nitrogen and ring-opened species has been investigated via ab initio molecular orbital theory at the MP2/6-31G(d,p) level. In most cases the decomposition is calculated to be a concerted but asynchronous process on the closed-shell singlet ground state of the azides. The five-membered ring is in fact found to be intact at the transition state. The activation barriers for the decomposition are found to decrease in the order: thiophenes>furans>pyrroles. Decomposition of phenyl azide is found to be more difficult to achieve than those of five-membered heterocycles. The higher energy demand for phenyl azide decomposition is probably due to smaller charge transfer between the azido group and the ring when compared to those of five-membered heterocycles during the decomposition. The stability of the transition state due to an internal electrostatic force was also found to be larger in the case of the five-membered heterocycles. The fate of the different products has not been considered. (C) 1997 Elsevier Science Ltd.