Coal combustion for energy production is the main source of mercury emission in Poland and several other countries. One of the pre-combustion methods to reduce mercury emission is coal cleaning by mild pyrolysis. Eighteen steam coals from the Upper Silesian Coal Basin in Poland have been used to study the thermal treatment in a developed system, based on the common MA-2 mercury analyser. Three groups of mercury compounds, sequentially decomposing during thermal treatment, were defined. The total mercury removal from the steam coals was in the range from 28% to 97% at 375 degrees C. The significant differences in the coals' ability to release mercury in the thermal treatment process were demonstrated. The best coals for thermal cleaning should contain compound with a low decomposing temperature (like HgCl2). There was no significant correlation found between basic parameters of coals (content of volatile matter, ash, sulphur and chlorine) and the amount of thermally removed mercury. The obtained data imply the need for careful selection of coals for the mild pyrolysis process. (C) 2015 Elsevier B.V. All rights reserved.