The deactivation of three different catalysts used in the cracking of high density polyethylene (HDPE) has been compared. The catalysts used are HZSM-5, Hβ and HY zeolites agglomerated with bentonite and alumina. The reactions have been carried out in a conical spouted bed reactor at 500 °C, and plastic (high density polyethylene) has been fed in continuous mode (1 g min−1) for up to 15 h of reaction. The HZSM-5 zeolite catalyst gives way to high yields of C2–C4 olefins (57 wt%) and, moreover, it is the one least influenced by deactivation throughout the run, which is explained by the lower deterioration of its physical properties and acidity. The results of temperature program combustion and transmission electron microscopy show that coke growth is hindered in the HZSM-5 zeolite pore structure. The high N2 flow rate used in the conical spouted bed reactor enhances coke precursor circulation towards the outside of the zeolite crystal channels.