The thermal degradation of poly(vinyl chloride)/acrylonitrile-butadienestyrene (PVC/ABS) blends of different compositions was investigated by means of isothermal thermogravimetric analysis at temperatures of 210 degrees-240 degrees C in flowing atmosphere of air. The Flynn equation, the method of stationary point, and kinetic equation using the Prout-Tompkins model proved to be satisfactory in describing the thermooxidative degradation in the range of 5-30% conversions. The apparent activation energy E and preexponential factor Z were calculated for all compositions of PVC/ABS blends. The ratiosE/ln Z are constant for pure and modified PVC and point to the unique mechanism of degradation process. Upon increasing the ratio of ABS in the PVC/ABS blend up to 50%, only the rate of the process is changed; the mechanism remains unchanged.