One sub-bituminous coal and two bituminous coals were subjected to the combustion and pyrolysis by slow heating to a temperature ranging 550-1150 degreesC. Leaching of raw coals, ashes and chars with dilute HCl and HNO3 was carried out, and leachate concentrations of major and trace elements were determined. Such a comparative leaching method was validated for characterizing the modes of occurrence of trace elements in coal and their transformations upon heating. Leaching results suggested that Be, V, Co, Cr and Ni were partially associated with organic matter, and As was partially associated with pyrite. During the ashing at 550-750 degreesC, the organically associated trace elements in coal formed some acid-soluble species. After the ashing at 1150 degreesC, Be, Co, Cr and Ni, together with Mn, Zn, and Pb, were immobilized in ash against leaching, whereas As was not immobilized. After pyrolysis, the organically associated trace elements in chars remained insoluble in both acids, and some HNO3-soluble As in coal turned to a HNO3-insoluble species. (C) 2003 Elsevier Ltd. All rights reserved.