Combustion residues in general and MSWI fly ashes in particular form a major environmental problem as they are polluted with heavy metals. The heavy metals can be removed from the fly ash by leaching with e.g. the acid wastewater obtained in the wet scrubber of the air pollution control system. The remaining fly ash can be landfilled or valorized e.g., a construction material. An understanding of the leaching reactions and of the factors that influence leaching is very important for the treatment of fly ash. Therefore, the composition of the fly ash was determined and the influence of pH was examined. In this paper, the results of experimental leaching tests of fly ash are compared with computer calculations of the thermodynamic equilibrium of the leaching solution-fly ash system. The computer program MINTEQA2, used for this purpose, allows to predict the metal concentrations in the leaching solution, the minerals that precipitate, and the pH of the leaching solution at equilibrium. Also a sequential extraction procedure was performed on the fly ash. The procedure can be used to investigate which leaching conditions are necessary to obtain a good extraction of the heavy metals. Comparison of the experimental and calculated leaching data with the results from the sequential extraction procedure allowed verifying the accuracy of the sequential extraction procedure. (C) 2000 Elsevier Science Ltd. All rights reserved.