Abstract: Azidohetarenes have found large interest because of its reactive azido group which gives a number of reactions which can be used synthetically [1]. We have prepared previously a number of hetaryl azides [2] having reactive substituents such as phenyl-, acyl-, hydrazono- and nitro groups in ortho-position which were used as synthons for thermolytic induced heteroelectrocyclic ring closure reactions. In this contribution we studied the thermal behavior of azidoarenes and azidohetarenes with and without reactive ortho-substituents to get insights in the thermal properties of these compounds which are interesting and important for thermal reactions.