In the course of this PhD thesis the high temperature corrosion behavior of commonly used construction materials was evaluated to find the most suitable reactor material for a thermal cracking process of post-consumer plastics. Materials ranged from low alloyed ferritic Fe-Cr steels up to high alloyed austenitic Fe-Cr-Ni steels and were, in general, tested between 420 and 580 °C, at laboratory scale. Test gas atmospheres contained 3.8 vol% HCl and a varying amount of H2S, which was either 0.02 vol%, 0.2 vol% or 2 vol%. Thus, the impact of different H2S levels on the high temperature corrosion behavior of various alloys in reducing HCl containing atmospheres could be studied. Tested materials were investigated by means of metallography, SEM/EDX and XRD measurements. In addition, the corrosion rates of the alloys were determined after the corrosion experiments. Furthermore, several thermodynamic calculations were performed with the software Fact Sage 8.0. Whit these data, high temperature corrosion mechanisms in reducing atmospheres containing HCl (3.8 vol%) and varying amounts of H2S (0.02 vol% - 2 vol%) were developed for ferritic and austenitic alloys at different temperatures. In general, an increase of the H2S content in the given chlorine containing atmospheres lead to an accelerated corrosion of all materials. However, at lower testing temperatures this effect disappeared and also the ferritic material, which in general revealed the poorest corrosion resistance, showed a comparable performance to the austenitic materials.