The thermal and catalytic degradation of high- and low-density polyethylene (HDPE and LDPE, respectively) and waste plastic film (polyethylene-based plastic wastes) were analyzed through simultaneous thermogravimetry and differential scanning calorimetry in inert atmosphere. Catalytic degradation was performed using MFI (HZSM-5) zeolites. Although the catalyst induces a large decrease of the degradation temperature for polyethylene, it has a smaller effect on the waste plastic (WP) degradation temperature. To check the activity of the catalyst after its use in the WP degradation, experiments were conducted with fresh HDPE which confirmed a significant loss of catalytic activity. Mixtures of WP with large paraffin were also analyzed (nC50). The results show that the presence of the hydrocarbon in the mixture grants some protection to the catalyst, allowing it to retain part of its activity during the process even in the presence of the waste contaminants. These findings suggest that larger hydrocarbon: waste plastic ratios promote higher protection to deactivation and that WP coprocessing with oil is feasible. Gas chromatography was used to analyze the products formed in the catalytic degradation of the WP in the presence C50 hydrocarbon at a ratio of 1:12, consisting mostly of light products in the range of C2 to C8 hydrocarbons.