An investigation was made of the thermolysis of poly(arylate-arylene sulphonoxides) containing different amounts of oligoarylene sulphonoxide blocks in a wide temperature range in open and closed systems. It was established that, at relatively low temperatures, the main process of their transformation is structure formation of the polymer; at elevated temperatures (above 400°C), degradation processes predominate over structure formation. It was established that structure formation can be used as a method for increasing the thermal stability of the polymers investigated.