AbstractExposure tests were carried out to study the reactivity of filter and cyclone ashes from biomass combustion plant in the atmospheres of biomass combustion with variations of H2O and HCl‐content for 360 h at 535 °C. The changes of C‐, Cl‐ and S‐content of the exposed ashes were analyzed by inductively coupled plasma atom absorption spectroscopy (ICP‐AAS). The analysis of untreated filter and cyclone ashes showed that the cyclone ash was more aggressive than the filter ash. As a result, in the HCl containing atmosphere the alkali chloride, which can be formed from alkali carbonate and hydroxides as well as oxides, can additionally accelerate the oxidation process. In the HCl free atmosphere at increased H2O content, alkali carbonates and hydroxides can be formed, so enhanced alkali chloride induced oxidation will not occur. Furthermore, thermodynamic calculations between alkalis and gases were conducted.