In order to gain deeper insights into aerosol formation processes during MSW combustion, test runs with a specially developed high-temperature aerosol measurement and sampling device, the so called high-temperature impactor (HTI), as well as subsequent chemical analyses of the particles sampled have been performed at a real-scale plant. The results show that aerosol formation during MSW combustion is based on the volatilisation of minor amounts of Si-, Ca-, Mg- and Al-compounds followed by nucleation in the fuel bed region which is then followed by excessive condensation of alkaline metal sulphates and especially chlorides in the radiative and the convective boiler sections. At lower flue gas temperatures in the superheater as well as the economiser section also the condensation of heavy metal (Zn, Pb) chlorides provides contributions to aerosol formation. (C) 2011 Elsevier B.V. All rights reserved.