Thermal stability of samples of polychloral prepared with different initiators and treated with various endcapping agents was studied using differential thermogravimetric analysis (DTG). Four temperature regions at which maxima of DTG curves occur were identified. On the basis of the known mechanism of initiation and the possible mechanism of termination, the structures of both initial and terminal endgroups of individual polychloral samples were elucidated which enabled the identification of the specific temperature region of thermal degradation of polychloral molecules having a specific type of endgroup. These results were confirmed by studies of the DTG curves of chloral-isocyanate copolymers containing stable terminal fragments with a high content of urethane linkages.