The behaviour of HZSM5 and HUSY zeolites during the decomposition of low density (LDPE) and high density polyethylene (HDPE) has been studied by TGA. Zeolites have been thoroughly characterized in order to elucidate the structures responsible for the observed behaviour. The reduction in the decomposition temperature is initially very high with HUSY for both polyethylenes while for HZSM5 is much greater with the branched LDPE than with linear HDPE. To explain the relative activity of these zeolites and the differences found for HZSM5 with both polyethylenes the polymer structure, the zeolite acidity and its textural properties have to be considered. The progressive deactivation behaviour of the catalysts has also been studied. HUSY shows a fast deactivation while small pore size in HZSM5 prevents coke deposition. Completely deactivated zeolites still show a considerable number of apparently unaltered acid sites. (C) 2004 Elsevier B.V. All rights reserved.