The paper discusses the issues related to the change of volume of the reagents during the process of combustion. The formulas that were necessary to make the calculations were derivedfor liquid and gaseous fuels. Based on the formulas, theoretical calculations of the change of the kilo-mole number for various fuels depending on the mixture composition were performed. The results of the calculations were presented in the tables. Further, the influence of volume contrac­tion on the parameters of the combustion process was analyzed. In the conclusions, a comparison of the influence of the previously described phenomena on the individual fuels was performed in relation to their chemical composition and the composition of the mixture.