Poly(vinylidene chloride) (PVDC) was decomposed by thermal treatment to 800 degreesC under inert atmosphere, using themogravimetry-mass spectrometry (TG-MS). The evolvement of hydrogen chloride was still observed at 800 degreesC for the degradation of PVDC. The dehydrochlorination process was divided into two stages according to difference in the formation rate of hydrogen chloride and trichlorobenzene. Dehydrochlorination was accompanied by the evolvement of trichlorobenzene during the first stage below 380 degreesC, but trichlorobenzene was not observed during the second stage above 380 degreesC.