This work deals with the investigation of the conditions under which a spontaneous degradation municipal waste plastic containing poly(vinyl chloride) (PVC) occurs during the dechlorination on an experimental scale by batch operation using a glass reactor, paying particular attention to the gas-solid reaction between the hydrogen chloride deriving from the PVC and the aluminum foil contained in the waste plastics. On the basis of observation of the effect of the rate of heating and the mode of contact between the PVC and aluminum foil on the change in the plastic melted level in the reactor, we were able to reproduce the spontaneous degradation of plastics with the production of a waxlike substance at around 593 K and at atmospheric pressure. The reason for the occurrence of spontaneous reaction was concluded to be the formation of "hot spots" (localized heating) originating from the heat of reaction between HCl generated from PVC and Al foil which eventually triggered the drastic thermal degradation of melted plastics. The possible conditions for the occurrence of the unusual reaction were discussed.