Thermal decomposition of commingled plastics comprising high and low density polyethylene, polystyrene, polypropylene and poly(vinyl chloride) was investigated under vacuum conditions by dynamic thermogravimetric analysis in the temperature range of 25–600°C. The mixtures were representative of the major polymeric materials found in a municipal plastic waste (MPW) stream. The study focused on the role of each polymer in the stabilization or destabilization of other interacting plastics, as well as the fate of chlorine produced from PVC. First, the decomposition of each single plastic was investigated. Then the binary combination of all above polymers and finally the multicomponent mixture similar to a MPW sample were studied. Two approaches have been applied to study the interaction between components in the commingled plastics. The first approach involved a comparison of the experimental curve of the decomposition of the mixture with a calculated non-interacting component decomposition curve based on the behavior of the individual polymers. The second approach involved a comparison of the kinetic parameters of each plastic in the mixture as determined by two different methods: (1) fitting the single polymer decomposition curve and (2) fitting the peak corresponding to the pyrolysis of one particular polymer on the mixture pyrolysis curve. The results obtained indicate that some interactions occurred during MPW pyrolysis, mainly at high temperatures (>375°C). PS and PVC appear to be the plastics responsible for the interactions through their intermediate pyrolysis products. The chlorine from PVC in the mixture is released almost completely before 375°C, when the conversion of the MPW to pyrolysis products has reached ca. 13%.