The behaviours of three mineral elements (Na, Fe and Si) during coal combustion in the temperature range 400-2200 K, pressure 1 x 10(5) Pa, under both oxidizing and reducing conditions, have been studied in detail by a thermodynamic equilibrium analysis. The effects of atmosphere and temperature on the speciation of mineral species are discussed. It was found that the combustion conditions greatly affect the major speciation of mineral, since it influences, to some extent, the vaporization of theses elements; When the temperature is above 1700 K, Na(g), NaOH(g), Fe(g) and SiO(g) are dominant gaseous species. The dominant species of mineral in flue gases depend on both the combustion conditions (reducing or oxidizing) and the type of coal. (C) 2003 Elsevier B.V. All rights reserved.