Thermal decomposition behaviour of phosphiniminocyclotrithiazenes [R=(i) phenyl-, (ii) piperidino-, (iii) morpholino-and (iv) N-methylpiperazino-] have been studied for the first time by employing thermogravimetric (TG), differential thermal (DT) and differential scanning calorimetric (DSC) methods. These heterocycles which do not show any melting behaviour begin to decompose above 130 degrees C by following different pathways. The study indicates the possibility of obtaining new products by controlled solid-state pyrolysis method. Thermo-chemical parameters for the decomposition process have also been evaluated.