The thermal degradation process of poly(vinyl chloride)/acrylonitrile-butadiene-styrene (PVC/ABS) blends was investigated by dynamic thermogravimetric analysis in the temperature range 50-650 degrees C in air. The thermooxidative degradation of PVC/ABS blends of different composition takes place in three steps. In this multistep process of degradation the first step, dehydrochlorination, is the most rapid. The maximal rate of dehydrochlorination for the PVC blends containing up to 20% ABS-modifier is achieved at average conversions of 23.5-20.0%, i.e., at 13.5% for the 50/50 blend. The apparent activation energies (E = 103-116 kJ mol(-1)) and preexponential factors (Z = 2.11 X 10(9)-3.45 X 10(10) min(-1)) for the first step of the degradation process were calculated after the Kissinger method.