In the study, the enthalpy of formation of a complex chemical compound, such as coal, was defined as the difference between the experimentally determined heat of combustion and the thermodynamically calculated heat of combustion of the elementary reactants. The boundary conditions for the approach were defined by the enthalpy of formation of graphite; thus, the aforementioned method should produce a value of zero for graphite. Using the developed correlation for the enthalpy of formation, a model of coal classification was developed based on this thermodynamic quantity, which reflects the structure and technological suitability of coal. According to the analysis of the enthalpy of formation with respect to the composition of coal, the enthalpy of formation may have negative or positive values, depending on the type of fuel. Furthermore, changes in the formation enthalpy are continuous but correspond to different chemical structures. The following values for the enthalpy of formation were obtained: anthracite = +250 kJ/kg, peat < -3200 kJ/kg and medium volatile bituminous coal similar to zero. (C) 2012 Elsevier Ltd. All rights reserved.