This paper describes a new treatment process to reduce leaching of trace elements (Pb, Cd a.o.) and salts from waste incinerator air pollution control (APC) residues. The process involves mixing of the APC residues with a ferrous sulphate solution and subsequent oxidation of the suspension. The treated product has a significantly reduced content of salts and the heavy metals are bound by the iron oxides formed. The process has been tested on a semidry gas-cleaning residue and a fly ash. The treated products were characterized by leaching tests including pH-static tests. The generated wastewater contained relatively low concentrations of trace elements (e.g. Pb: 29–162 μg/l and Cd: 3.3–7.5 μg/l), but high concentrations of salts (e.g. Cl, Na, K, and Ca). The treatment process reduces the leaching of Pb from the residues by more than two orders of magnitude at fixed pH. Likewise, the Cd leaching was significantly reduced. Similar results were observed for Zn and Cu. The effect on elements that form oxyanions is more uncertain and in the current process there is no reduction in the release of Hg.