Plastic waste can be successfully recycled because it can be used as a very valuable secondary raw material and it is also suitable for thermal treatment due to its high energy value. Pure homopolymers are relatively easy to recycle while the mixtures of polymers are great challenge, and therefore for this type of waste as a potential treatment is pyrolysis. In this paper commonly used polymers were studied: high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) and theirs mixtures. The thermal stability of pure homopolymers and their blends (Mix 1-5) as well as blends in the presence of Fe/Al2O3 catalysts (Mix / F 1-5) was investigated, using thermogravimetric analysis (TGA). The thermal stability of these samples was investigated by determination of initial degradation temperature (T90), temperature at maximum degradation rate (Tmax) as well as carbonized residue (mr) and maximum degradation rate (rmax). Also, the kinetics of their thermal decomposition was monitored, with measurements performed at 4 different heating rates (5, 10, 15, 20 °C/min). The kinetics of thermal decomposition of the investigated samples was analyzed using the KAS model and the mechanism of decomposition was considered, i.e. the occurrence of parallel decomposition reactions was studied. The activation energies (Ea) of thermal decomposition were determined by the isoconversion KAS method. From the results it can be concluded that with the increasing heating rate increase of thermal stability for all samples is observed. From the results of kinetic analysis and activation energy (Ea) the occurrence of parallel, sequential and reversible reactions in the process of thermal decomposition has been observed as a consequence of the different composition and structure of the polymers. Due to the occurrence of parallel reactions, the values of Ea for Mix /F 3, 4 and 5 do not correspond to the actual activation energies, they are slightly decreasing or increasing.