The kinetics of the steam reforming of high-density polyethylene (HDPE) pyrolysis volatiles is studied on a Ni commercial catalyst in a fluidized bed reactor in line with the pyrolysis reactor (a conical spouted bed reactor at 500 degrees C). Steam reforming reactions have been carried out under the following conditions: temperature of 600-700 degrees C and space time of 0-16.7 g(cat) min g(HDPE)(-1). On the basis of the composition of HDPE pyrolysis volatiles, a kinetic scheme is assumed with four reactions (C5+ hydrocarbon reforming, C-2-C-4 hydrocarbon reforming, CH4 reforming, and water-gas shift reaction). Moreover, the kinetics of the deactivation has been quantified with an expression dependent on the C5+ hydrocarbon concentration (coke precursors). Calculation of the kinetic parameters is conducted by nonlinear multiple regression, fitting the experimental data to those calculated by the mass conservation equations for each component in the reaction medium. The overall kinetic model proposed accurately describes the evolution of the main products (H-2, CO2, and CO) with time on stream in the range of operating conditions studied.