Currently only 20% of the fly ash produced in Korea is utilised for industry, and the remainder is disposed as waste in landfill sites. Both anthracite and sub-bituminous coals are burnt in Korea. Fly ash and coal samples were collected from five different coal-fired power stations in Korea and analysed for their chemistry and mineralogy. Batch leaching tests were also carried out to investigate the leaching behaviour of selected fly ashes. The fly ash leachate chemistry was compared with the groundwater taken directly from the monitoring well installed in one of the power stations. The anthracite coals contain illite, pyrophyllite and kaolinite whereas kaolinite is the representative clay mineral for the sub-bituminous coals. Anthracite coals were higher in Si, Al and K than the sub-bituminous coals, reflecting higher mineral matter contents in the anthracite coals. Mullite and quartz are the main mineral phases for two different types of the fly ashes, with some iron oxides. The chemical compositions of the anthracite and sub-bituminous fly ashes are comparable with each other, except for extraordinary high concentrations of Cr for one anthracite fly ash. Most of the trace elements in the ash were enriched in the finer fraction, indicating surface associations. Although, some elements including Na, K, Ca and Cu were released rapidly in the initial stage of leaching, measurable amounts of metals were still detectable in the fly ash leachate treated several times with distilled water. Such leaching behaviour indicates slow and long-term leaching of elements associated with the class fractions of the ash particle. This was confirmed by leaching of weathered fly ash, which had been disposed of for several years. Comparison of the ash leachate, treated with 0.1N-HCl, fly ash slurry in the ash pond and the groundwater indicate the influence of the ash leachate from the ash disposal mound on the groundwater composition. (C) 2002 Elsevier Science Ltd. All rights reserved.