Within the last years, attention has been focused on the development of clean coal technologies, based on the pulverized coal fired once-through boiler technology and the circulating fluidised bed combustion (CFBC) systems. The environmental problems provoked froth those wastes due to their toxic trace element contents necessitate their detailed characterisation. Within the scope of this concept, two different coal types were used in a series of experimental trials. Fly and bottom ash samples were collected and characterised in terms of morphology, mineralogy, leaching and toxicity behaviour. According to the results, toxic trace elements are preferentially concentrated in the fly ash particles since they presented the smaller particles size. However, the chemical analysis of the ash leachates showed that are acceptable for safe disposal, since none of them exceeds the maximum EPA, limits. Additionally, the Microtox toxicity test proved that fly ash leachates, which presented the higher heavy metals concentrations, caused the higher toxic effects. (C) 2007 Elsevier Ltd. All rights reserved.