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Thermodynamic analysis of carbon gasification process in the presence of moisture was carried out. The chemical process was displayed by the system C – O – H with the ratios of elements in it: 1:1:2 and 1:2: 2. To work out the methods of research and verification of the results, we used a well-studied subsystem C – O. The initial array of processed data was presented by the contents of chemical components C, CO, CO2 , CH4 , H2 and H2O calculated by TERRA program. There is no single chemical reaction in the C – O – H system, so the full operating temperature range of 298 – 1400 K was divided into three characteristic areas, and each of them was analyzed separately. By comparing the numerical values of the components contents at the regions’ boundaries, we determined changes in their values during the transition from one region to another. These values were multiples of stoichiometric coefficients of the expected chemical reactions. Thus, the problem with establishment of the chemical reactions’ type was solved. But two areas of three identified reactions were complex containing more than four components. Therefore, their decomposition was performed on the basis of three more simple and characteristic reactions for these areas. As a result, the total number of reaction varieties was reduced to four – two main reactions of carbon gasification (C + 2Н2О = CO2 + 2Н2, C + CO2 = 2СО) and two reactions of formation and decomposition of methane (2C + 2Н2О = CH4 + CO2 , CH4 = C + 2Н2 ). At the same time, the proportion of each reaction in the total chemical process was determined by the balance coefficients β.The type of chemical reactions provides the necessary information about content of the system components only at the regions’ boundaries. A quantitative assessment of the chemical process within the regions can be obtained by determining the temperature dependence of the reaction coordinates on Gibbs energy of the reactions and the pressure – ξ(Т) = f [ΔrG°(Т), Р]. The coordinates of reactions ξ in combination with the balance coefficients of reactions β allow us to calculate not only the content of reagents and reaction products at any moment of reactions, but also the conditional temperatures of the beginning and end of the reactions themselves. No coefficients and parameters of the fitting character were used in the calculations. The average absolute error of the quantitative description of the results of machine simulation of the system C – O – Н – is less than 0.02 mole (per 1 mole of carbon), and for the subsystem C – O it is almost zero.