One of the most serious environmental problems is plastic pollution. The production of single use plastic products goes beyond the world's handling capabilities. Most plastics do not degrade, instead, they slowly decompose into smaller particles, called as microplastics and nanoplastics. The lifespan of plastics should be prolonged by reusing and recycling as many times as possible. Since plastics are generally high calorific value products, incineration of plastic polymers is way to treat waste while both exploiting the energy of the waste material and reducing the amount of solid material to be landfilled. In this study, samples of the tested polymers were obtained from Rotoplast d.o.o. in Sveta Nedelja. The following polymeric materials were selected for analysis: amorphous polyethylene terephthalate/polyethylene (APET/PE), polyethylene terephthalate/biaxially oriented polypropylene (PET/boPP), polyethylene terephthalate-low-density polyethylene metallized (PET/PE-LD met), polyethylene terephthalate/polyethylene laminated (PET/PE lam). Two commercially available catalysts were used in this analysis, nickel(II) chloride (NiCl2) and iron(III) oxide (Fe2O3). Thermal degradation of the tested samples was performed by thermogravimetric analysis (TGA) on the TA Instrument Q500. Measurement was performed in an internal atmosphere of nitrogen (N2) at a flow rate of 60 ml/min, in the temperature range from 40 to 600 °C, at different heating rates: 5, 10, 15 and 20° C/min. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves were determined by measurement. The assessment of the occurrence and presence of parallel, consecutive and reversible reactions in the thermal degradation process was monitored. The kinetics of thermal degradation and the activation energies (Ea) of thermal degradation of the investigated samples were determined by the isoconversional Kissinger-Akahira-Sunose (KAS) and Friedman methods. From the obtained results it can be concluded the Fe2O3 catalyst is not suitable for mixtures of such components because the parallel reactions slowed down the thermal degradation. Meanwhile, the NiCl2 catalyst is more suitable than Fe2O3 because it reduces the activation energy, which means a lower temperature and less energy is required.