The master thesis deals with expanded polystyrene (EPS) and its thermal degradation. Theoretical part of thesis consists of overview of chemical, physicochemical and physical properties of polystyrene and its use in construction. Next important parts are physical and chemical changes occurring during combustion and fire of EPS. The thermal degradation of this polymeric material provides an opportunity to examine the phenomena that occur during this process. The main goal of the thesis is to compare the thermal degradation of the selected polystyrene type in the air and in an inert atmosphere by selected methods of thermal analysis performed at different heating rates. To perform the experiment, we used thermal analysis methods, such as thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). Using a thermal analyser, the individual samples were gradually exposed to thermal heating in nitrogen and in the air. There were made new findings about the influence of the atmosphere anddifferent rate of weight loss to the overall course of thermal degradation of EPS, from the evaluated TG, DTG and DSC curves. Mutual comparison of the results is a practical contribution to assessing the behaviour of EPS in thermal loads by the fire.