Thermodynamic calculations of the chemical composition of the reactions products of interaction between the mineral substances of coke ash and elementary carbon have been made. The calculations were performed using the REACTOR1 and REACTOR3 software based on the Gibbs free energy minimization method. The results of the chemical analysis of ash derived from one of the Polish cokes were taken into consideration. The number of compounds forming from the components of coke ash during heating in a stream of gases of highly diverse reducing ability reflect the complexity of physicochemical phenomena. These phenomena may be, to a considerable extent, responsible for the destruction and weakening of the proper structure of coke. Ranges of temperatures of occuring chemical compounds depending on the gas phase composition used in the calculations were given.