The paper deals with mercury release during thermal treatment of Polish coals. Three coal types (two hard coals and a lignite) from Polish coal mines supplying fuel for the power generation industry were used in the experiment. The release of mercury was investigated in a fluidized bed reactor heated to moderate temperatures 170 degrees C-410 degrees C. The thermal treatment of mercury was conducted in various atmospheres (air, nitrogen and carbon dioxide). Except for mercury concentration in the flue gas the emission of CO, CO2, and O-2 was also determined. The experimental results indicated that the mercury release from hard coals strongly depended on the gas atmosphere. The highest mercury release was determined in CO2 atmosphere, while the Hg release in air or nitrogen gas was significantly lower. In the case of lignite no effect of gas type on the mercury release was stated. The results of the investigation also indicated that regardless of the coal type and the process temperature the loss of chemical energy of the coal samples, calculated from the emission of CO and CO2 was less than 5%. (C) 2013 Elsevier B.V. All rights reserved.