The thermal and catalytic pyrolysis of LDPE and HDPE over HZSM5 and HUSY have been studied under dynamic conditions in a batch reactor, The products evolved have been analyzed, and results have been compared with those found in the bibliography. Particular attention was paid to any coincidences and differences encountered. The yields of gases, liquids and waxes for all the systems employed are presented, along with the composition of each fraction. The general carbon number distribution is shown, as well as the carbon number distribution for the main compounds present in the gaseous and the liquid fractions. Finally, the 10 major compounds obtained for each system are described. (C) 2008 Elsevier B.V. All rights reserved.