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Abstract: Solidification/stabilization (S/S) treatment process is the most economic and popular method to treat fly ashes generated from municipal solid waste incinerators. The objective of this study was to re-evaluate the leaching tests of field solidified samples, in order to re-confirm the previously successful metal leaching test performed in the laboratory. Different batches of freshly solidified samples were taken from a S/S plant, and then cured in the laboratory up to 180 days. The toxicity characteristic leaching procedure (TCLP) and compression test of samples were performed. After the second day curing, the TCLP test results for some samples indicated an acceptable Pb level (from 70 mg/L of the original fly ash to 3 mg/L or less) and the compression strength could reach 10 to 40 kg/cm2. These values could pass the Taiwan’s regulation requirements of 5 mg Pb/L and 10 kg/cm2 strength. Unfortunately, about one third of samples failed to meet the TCLP and/or compression strength requirements even after 180 days curing period, e.g., with Pb as high as 30 mg/L and strength as low as 8 kg/cm2. It was thought that during the fly ash storage period, the high humidity in Taiwan would somehow cause Pb-containing particles enmeshed and aggregated; hence, these “hot spots ” would interfere with the solidification reactions between fly ash and cement. The solidified samples that failed to meet the leaching standard were crushed and milled to different particle sizes (from 0.125 to 9.5 mm), then re-solidified with same cement reagents. The Pb concentrations in the TCLP test for most samples exceeded the standard again. It appears that the so-called hot spots were not responsible for the unacceptable leaching results. The addition of sulfide base additive, however, is able to reduce leachable Pb levels, apparently due to the formation of insoluble PbS