Polyacrylonitrile (PAN) is an important industrial polymer. Its thermal decomposition is exothermic and the onset temperature is close to its drying temperature in industrial production. The thermal decomposition process of PAN is a self-heating type and can produce many toxic products. Thus there is a severe thermal hazard associated with its drying process. In the present study the products of thermal decomposition at various heating rates and isothermal temperatures in the presence and absence of oxygen are evaluated using simultaneous TG-MS technique to provide an insight into the mechanism of their liberation and to obtain qualitative knowledge of the associated hazards. The identified gaseous products liberate simultaneously in the temperature range 150-400 degrees C and their evolution is dependent upon the heating rate and temperature. The principal reaction at lower heating rate is dehydrocyanation. Carbon dioxide liberation is observed, in addition to other products, in the presence of an oxygen-inert mixture atmosphere.