Fly-ash scale on boiler tubes in waste incineration plants can significantly decrease the lifetime of high-temperature alloys in the boiler due to corrosion. These reactions occur between sulphates and chlorides of the fly-ash and the metallic substrate. The influence of the fly-ash deposits on corrosion reactions is not known in detail. However, it is well established that tubes without deposits have a much longer lifetime. Several reactions take place between the corrosive flue gas containing HCl and SO 2 and the deposit. The phase composition of the deposit strongly depends on the composition of the gas phase. In order to study the effect of gas composition on the mineralogical phases in fly-ashes, deposits were recovered from boiler tubes of a waste incinerator and treated in a thermobalance (TG) at 500 degrees C with atmospheres resembling the typical conditions inside waste incineration plants (500 vppm HCl, 250-1000 vppm SO 2 , 500 vppm HCl+500 vppm SO 2 , 500 vppm HCl+1000 vppm SO 2 ). After the TG-runs, the reacted material was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron microprobe (WD-EPMA) and scanning transmission electron microscopy (STEM). The original fly-ash is composed of various sulphates (CaSO 4 , K 3 Na(SO 4 ) 2 , K 2 Pb(SO 4 ) 2 , K 2 Ca 2 (SO 4 ) 3 ), alumino-silicate glass and iron oxides. The introduction of a HCl-atmosphere at 500 degrees C leads to a considerable weight loss of the sample and the formation of chlorides (NaCl, KCl). The weight loss is due to the removal of heavy-elements (Pb, Zn) as volatile chlorides and the production of SO 2 from the chloridation of sulphates. In the SO 2 -containing atmosphere, a weight gain occurs due to the uptake of SO 2 /SO 3 from the gas phase, accompanied by the formation of pyrosulphates. In atmospheres containing both HCl and SO 2 a minor weight loss occurs due to the uptake of SO 2 /SO 3 from the atmosphere and the reduced volatilization of heavy-metal chlorides. Both atmospheres (HCl, SO 2 ) decompose complex sulphate phases and initiate the crystallisation of endmember sulphates. These results have implications for the choice of corrosion-resistant high-T alloys in municipal waste incinerators and for the performance of flue-gas purification procedures.