The detailed mechanism of high temperature chlorine corrosion, the dominant cause of corrosion in a municipal solid waste incinerator (MSI), has still to be clarified. Upon its way through the boiler the raw gas is subject to various physical and chemical processes and interactions. Of these, sulfation of chlorides is supposed to have the major impact on chlorine corrosion. The physical and chemical mechanisms of corrosion were investigated at a municipal solid waste incinerator. Both the particulate and gas phases of the flue gas were chemically and physically analyzed during their way through the boiler, at temperatures from close to 1000C down to 200C. The raw gas composition was analyzed during normal operation and during the soot blowing cleaning routine. Additionally, operating parameters of the plant were varied, and deposition processes were evaluated with the aim to find out primary measures to reduce corrosion rates. The particle mass concentration exhibits a bimodal size distribution with maxima at approximately 0.5 mgrowing by duration of traveland 100 m. First results show that sulfation of the particles can be observed upon travel through the boiler and on the fouling. Sulfur-containing additives increased the sulfation of the particles during flight, though not to completion.