The pyrolytic reaction of chlorinated plastics such as poly(vinyl chloride) (PVC) releases hydrogen chloride (HCl). The HCl released from the pyrolysis, in turn, may affect the mechanisms of the pyrolytic reactions. In this study, the pyrolysis of high-density polyethylene (HDPE), being the principal component of plastics in the municipal solid waste (MSW), was carried out in the thermal gravimetric analysis (TGA) reaction system containing HCl gas. The results indicated that HCl inhibited the pyrolytic reaction of HDPE. The activation energies, pre-exponential factors, and reaction orders of the reactions corresponding to the various concentrations of HCl (C H C l ) were obtained in the range of the experimental conditions. The results of this study are useful for the utilization of HDPE as an energy resource and for determining the thermal degradation rate of HDPE in an incinerator.