High-temperature corrosion mechanisms in reducing atmospheres containing HCl (3.8 vol%) and a varying amount of H ₂S (0.02 –2 vol%) were developed for several alloys between 420°C and 680°C. These mechanisms are mainly based on practical observations and kinetic considerations—and less on thermodynamic data. This is due to the complexity of these mixed gas atmospheres, volatile corrosion products, and the ever-changing conditions within the corrosion layer, which made it not possible to predict and calculate the actual conditions in the corrosion zone. In this article, a detailed thermodynamic analysis of previously achieved corrosion mechanisms and experimental observations is presented. Correlations and deviations between thermodynamic calculations and practical findings are stated and discussed. The corrosion behavior of ferritic K90941, which performs worse than corrosion-resistant austenitic alloys, except for one test condition at 580°C in the atmosphere with 0.2 vol% H ₂S, is explained and supported by thermodynamic data. By combining experiments with thermodynamics, corrosion mechanisms in reducing HCl and H2S-containing atmospheres are explained.