Abstract Effects of copper and iron chlorides were studied on the thermal decomposition reactions of polymers using pyrolysis-gas chromatography/mass spectrometry. Copper and iron chlorides found to depress the radical transfer reactions of lower probability in polypropylene and polystyrene at 500°C through the interaction of the radicals and the transition metal ions. Cu(I) and Fe(II) chlorides enhance the production of aromatic and polyaromatic compounds from polyethylene at 1000°C promoting hydrogen elimination. Partial decomposition of the methyl ester side groups takes place in poly (methyl methacrylate) at 450°C, leading to chloromethane and carbon dioxide, when Cu(II), Fe(II) or Fe(III) chloride is present. Heterolytic cleavage of the methoxy group is presumably promoted by the electrophyl transition metal ions. Similar effect was proposed explaining the enhanced phenol evolution from phenol-formaldehyde resin, polycarbonate and epoxy resin at 500–600°C in the presence of Cu(II) and iron chlorides. The rupture of phenolic rings and the reduction of the phenolic hydroxyl groups leading to polyaromatic pyrolysis products from phenol-formaldehyde resin at 1000°C was reduced by Cu(II) and iron chlorides presumably due to a protective interaction of the phenol rings and the transition metal ions. Pyrolysis of polymers was performed also in the presence of PVC and copper or iron. The results lead to the conclusion that the chlorides of lower oxidation state are forming from PVC and copper or iron when co-pyrolysed with polyethylene or phenol-formaldehyde resin.