The purpose of this study is to test the effectiveness of mixing FeSO4 and Fe-2(SO4)3 solutions with municipal solid waste incineration (MSWI) ash at room temperature to prevent heavy metals in treated ash from being dissolved. After conducting toxicity characteristic leaching procedure (TCLP) tests nine times, the accumulation of extracted lead from treated scrubber ash was lower than regulated limits by the sixth test. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) studies of the ash revealed that the mixed solution reacted with the scrubber ash to form Ca4Fe9O17, which coated the surface of ash particles. (c) 2005 Elsevier B.V. All rights reserved.