The catalytic pyrolysis of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) over the HZSM5 catalyst in a batch reactor has been studied, and an analysis of the evolution of the products generated with temperature is presented. As expected, the catalyst favored the formation of gaseous compounds. The composition of the products obtained depended upon the polyethylene structure, especially at the beginning of the degradation process. Initially during the catalytic cracking of LDPE, olefins were the major compounds obtained in both the gaseous and condensed phases. As the temperature was increased, the composition of the gases remained constant, whereas in the condensed fraction, the olefins yield decreased, resulting in aromatics being the compounds predominantly generated. With HDPE, a greater quantity of paraffins and aromatics formed at the beginning of the decomposition process in the gaseous and condensed products, respectively, as compared to LDPE. As the temperature was increased, however, a more similar composition to that for LDPE was obtained.