An experimental and theoretical study has been carried out on the thermal degradation of large samples of high density polyethylene in an inert atmosphere. The experimental results show the existence of significant longitudinal temperature profiles inside the solid, which are basically due to its low thermal conductivity and the melting process. The polyethylene undergoes important volume variations during the experiments: expansion during the melting process and contraction due to pyrolysis. The mathematical model developed takes into account the volume variation and the melting process using the enthalpy method. The temperature profiles and the solid conversion are predicted, and good agreement with the experimental temperature results is obtained. Some discrepancies are observed in the case of solid conversion, probably due to the resistance to mass transfer of product gases in the solid.