The paper presents the results of a research on the effects of process parameters on the combustion of waste plastics. The experiments were carried out in a laboratory fluidized bed reactor. The temperature and the conditions of the process were changed during the experiments. The plastics were combusted continuously (autothermally), periodically with extra fuel (co-firing) and alone in a hot fluidized bed. During the combustion process of materials containing nitrogen (PA, ABS), while the bed temperature decrease, changes of emissions of nitrogen oxides, in particular an increase in the concentration of N2O, up to 250 ppm at ~730°C, were observed. During ABS combustion, emission of HCN was registered, at a maximum of 400 ppm. The presence of the supporting fuel (LPG) resulted in the stabilization and acceleration of the plastic samples degradation process. The rate of thermal decomposition of waste materials depended on its elemental composition and also the physicochemical properties.