High-density polyethylene (HDPE) was cracked over HZSM-5 and HY zeolites and the reaction was followed using simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) and was compared with the degradation of the same material in the absence of an added catalyst. The products obtained in the degradation reaction were analyzed by gas chromatography. The simultaneous use of the signals from the TG and DSC allowed an accurate description of the thermal and catalytic degradation of the polymer by application of a novel kinetic model that correlates the two signals that are measured. The kinetic parameters were estimated by fitting this model to the experimental data obtained by TG and DSC. For both zeolites, the polymer degradation takes place at lower temperatures when compared with pure thermal degradation. It was also observed that the two zeolites have a distinct influence on the product distribution.