For chemical recycling of waste plastics, HZSM-5, HY, and H-mordenite zeolites and silica-alumina were examined as catalysts for the degradation of polyethylene in a fixed-bed flow reactor system, and their activities and deactivation behaviors caused by coke deposition were studied. HZSM-5 catalyst was found to be very effective for the production of gasoline-range fuel oils mainly consisting of isoparaffins and aromatics and showed no deactivation due to a very low yield of coke deposited on the catalyst surface, whereas in the degradation of polystyrene a marked deactivation was observed (Uemichi et al. Kobunshi Ronbunshu 1993, 50, 887). Silica-alumina gradually deactivated as time on stream increased, but the degree of deactivation was less than expected from the deposition of a significant amount of cake, probably because the coke deposition in the large pores of the catalyst caused no marked influence on the diffusion of the decomposed fragments involved in the reaction. On the other hand, deactivations of HY and H-mordenite were striking; the latter was most abruptly deactivated, resulting in a marked decrease in the liquid yield. From the surface area measurements of the used catalysts, it was suggested that the pores of HY were sufficiently filled out with coke, while pore blocking by coke occurred in the unidimensional channels of H-mordenite.