Dissertation thesis entitled „Formation, characterization and analysis of the products of thermal degradation and combustion of polystyrenes in the internal fire“ defines properties and chemical structure of polystyrene, fire-technical characteristics and classifies the various types of polystyrene. The aim of this thesis is to analyse volatile products of thermal degradation and combustion of selected types of polystyrenes, specifically polystyrene flooring plates which are used like interior decorative boards. The analysis of the volatile compounds arised from the thermal decomposition of the material samples in the temperature range 60 – 300 ° C was carried out by the gas chromatography mass spectrometry method. Styrene, toluene, ethylbenzene, α-methylstyrene, benzene derivates were identified. We also focused on the quantitative analysis of the selected gaseous thermal degradation products and we confirmed significant influence of material and temperature on their concentration by two-way analysis of variance. The thermal stability of selected types of polystyrene flooring plates was determined by thermal analysis. We completed information about these materials by the results of the gel permeation chromatography method, we found out molecular weight changes and average degree of polymerization changes of thermally loaded samples in comparison to the original material.