The objective of this study was to investigate the leaching of assimilable silicon species from coal fly ash with distilled water, sea water and synthetic sea water at various fly ash/water ratios, pHs and temperatures. At the 1 g/100 ml fly ash/water ratio, less than 1 mg Si was found in 1 1 of aqueous slurries over the pH range 4-8 after 2 h at ambient temperature. The leaching was most effective at pH 10.5. At the fly ash/water ratio indicated, the pH of the suspensions decreased from 10.4 to 8.4 after 5 days. The pH of fly ash slurries in sea water varied only slightly over time as compared with that in distilled water. Generally, the leaching of assimilable silicon species with distilled water was more intense than that with the sea water.