Pyrolysis of high density polyethylene (HDPE) has been carried out in a conical spouted bed reactor (500 degrees C) and the volatiles have been reformed in-line over a Ni commercial catalyst in a fluidized bed reactor. The evolution of reaction indices (conversion, H-2 yield and other gaseous product yields) with time on stream has been studied under the following operating conditions: temperature, 600-700 degrees C; space-time, 8.3-20.8 g(cat) min g(HDPE)(-1), and steam/plastic ratio, 3-5. High initial conversion ( > 94.5%) and H-2 yields ( > 76.7%) are attained under all the operating conditions studied, with HDPE conversion and H-2 yield increasing when the three variables are increased, which is explained by the enhancement of the reforming reaction. However, a significant effect of operating conditions on catalyst stability has been observed. Thus, an increase in temperature, space-time and steam/plastic ratio decreases catalyst deactivation as a consequence of a lower rate of coke deposition on the catalyst due to both a lower C5+ coke precursor amount in the reaction medium and a higher gasification rate of the coke deposited.