Municipal solid waste (MSW) incinerator (MSWI) bottom ash and fly ash were used as landfill cover or were co-disposed with MSW to measure their potential metal-releasing and acid-neutralizing capacity (ANC) in landfill sites. Five lysimeters (height 1.2 m, diameter 0.2 m), simulating landfill conditions, were used in the experiment. Four contained either bottom ash (BA) or fly ash (FA) with BAM:SW ratios of 100 and 200 g L-1 and FA:MSW ratios of 10 and 20 g L-1, and the fifth was the control, which contained no ash. The lysimeters were arranged so as to contain four layers, with BA or FA placed on top of MSW within each layer. Each lysimeter was recirculated with 100 mL leachate using peristaltic pumps, and 100 mL of the leachate was collected weekly to measure the soluble metal concentrations. The results showed that the concentrations of soluble alkali metals measured in the leachate were in the order Ca > K > Na > Mg. In addition, the concentrations of soluble alkali metals of Ca and K collected from the lysimeters containing FA were found to be higher than the concentrations from the lysimeters containing BA. The concentrations of heavy metals (Cd, Cr, Cu, Ni, and Zn) were found to be < 1 mg L-1 except for Pb, which reached 2 mg L-1. These results suggest that for alkali metals there might be an ANC consistent with the results of an acid titration curve, which would provide suitable conditions for anaerobic digestion of the MSW in the landfill. Furthermore, heavy metals and trace metals were found in concentrations, which were too low to exert inhibitory effects on anaerobic digestion, and thus they could serve as micronutrients to exert beneficial rather than detrimental effects on landfill biostabilization. (c) 2006 Elsevier B.V. All rights reserved.