The catalytic cracking of high density polyethylene (HDPE) has been carried out at 500 °C in a conical spouted bed reactor with two catalysts prepared with HZSM-5 zeolites with SiO2/Al2O3 ratios of 30 and 80. The polyethylene has been fed continuously (1 g min-1) over 10 h to a 30 g catalyst bed. The results show the good performance of the conical spouted bed reactor in minimising the limitations of the physical steps of the process. The deactivation of the catalysts is very low and it is demonstrated that the moderation of the acidity is useful in modifying the product distribution. The SiO2/Al2O3 ratio increment involves a decrease in the total acidity and in the acid strength, resulting in a higher yield of C2-C4 olefins and that of the non-aromatic C5-C11 fraction, and a decrease in the yields of aromatic components and C1-C4 paraffins. The yield of the C2-C4 olefins obtained with the HZSM-5 zeolite catalyst with a ratio of Si/Al2O3 = 80 is 59.8 wt% (that of propylene is 29.6 wt%) and the yield of the gasoline fraction (C 5-C11) accounts for 32.1 wt%. The coke deposited on the catalyst has a heterogeneous nature and is constituted by two types of coke, which are deposited on the exterior and the interior of the crystalline channels of the HZSM-5 zeolite. The evolution of the coke is attenuated as the SiO 2/Al2O3 ratio of the zeolite is increased. © 2011 Elsevier B.V. All rights reserved.