The application of accelerated carbonation and natural carbonation to treat municipal solid waste incinerator (MSWI) fly ash was presented. The influence of reaction time and the liquid-to-solid (LIS) ratio, which both affect the reactivity Of CO2, was evaluated to determine their impact on the quality of carbonation. The optimum carbonation reaction of fly ash was found to occur when an LIS of 0.25 was used. Carbonation decreased the leaching of Pb, Cu, Zn and As, but increased the leaching of Cd and Sb. Based on the leaching of these six heavy metals, the optimum pH of the carbonated fly ash was 9.5-10.5. The release of soluble salts such as SO4, Cl and F changed little following carbonation, and their release occurred independently of pH. The release potential and leaching behavior of carbonated fly ash were further evaluated using the three-step sequential extraction procedure proposed by the commission of the European Communities Bureau of Reference (BCR). The results of the BCR analyses revealed that carbonation exerted a significant effect on the heavy metal fractions in steps 1 and 2, but little effect on the distribution of heavy metals in step 3 and residue fraction. Physical and chemical changes induced by carbonation were presented and discussed. (C) 2009 Elsevier B.V. All rights reserved.