Polystyrene is one of the oldest and most frequently used synthetic polymers. It has got a broad range of use. Among its shortages belong flammability and low resistance to degradation. Many works deal with pyrolisis and thermal disintegration of polystyrenes under different conditions and temperatures. The diploma thesis named „Characteristics of volatile products in the initiative phase of synthetic polymers“ concerns analysis of volatile products arising by the thermal load of the facade polystyrene EPS 70F at the temperatures ranging between 20°C – 200°C. Polystyrene had been thermally burdened and the newly-emerged volatile products were analysed using the HS-GC-MS device. During the thermal degradation of polystyrene there was mainly styrene coming into existence, which content was rising alongside the increasing of temperature. Except for styrene there were also ethylbenzene, toluene, xylene, oxidation products like benzaldehyde, acetophenone, phenol and by 200°C styrene dimers present in the degradation products.