The valorization of waste high density polyethylene (HDPE) for hydrogen production has been studied in a two-step process, comprising pyrolysis and subsequent steam reforming of the volatiles produced in the first step. Particularly, this work focuses on the deterioration mechanisms (sintering and coke deposition) of the Ni commercial catalyst used in the second step, as it conditions the overall process performance. Pyrolysis of HDPE has been performed in a conical spouted bed reactor at 500 °C, and the catalytic steam reforming of the pyrolysis volatiles, in a fluidized bed reactor at 700 °C. Deactivated catalyst samples were recovered at different values of time on stream, and characterized using XRD, N2 adsorption-desorption, SEM and TEM electronic microscopies, temperature programmed oxidation (TPO), Raman, FTIR and LDI-TOF MS spectroscopies. The results show that the deactivation is due to the sintering and encapsulation -by coke- of Ni. The former is inevitable within the current conditions, and the latter can be ascribed to the condensation of adsorbed precursors that evolve over time. Encapsulating coke is partially carbonized into filamentous coke with lower effect on catalytic deactivation and higher economic interest.