The chemistry and behaviour of the heavy metals As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn as they move from the furnace to the filter in a combustion plant for MSW (Municipal Solid Waste), has been investigated. Focus has been on the influence of varying MSW composition and operational parameters such as air/fuel ratio and temperature. It was found that only Hg is present in gas phase after boiling at 500 K. The availability of sulphur and chlorine largely influenced the condensing behaviour and chemistry for many of the heavy metals. One important observation is the shift from solid phase chlorides to sulphates at temperatures below 900 K for Ph and Zn, when increasing the sulphur availability, Ph and Zn sulphates being less corrosive in boiler systems than their respective chlorides. (C) 2004 Elsevier Ltd. All rights reserved.