Aviation in Europe is required to use fuels containing up to 2 wt. % of sustainable aviation fuels (SAFs). A better understanding of the impact of SAFs on the combustion process will be helpful in solving problems that may arise from the widespread use of these kinds of fuels. It was assumed that the reactivity coefficient αi and the activation energy could be a criteria for assessing the impact of SAFs on the combustion process. Based on DGEN engine tests, the following activation energy values of CO2 and CO formation reactions were obtained—Jet A-1: EaCO2/R=3480 and EaCO/R=982; A30: EaCO2/R=3705 and EaCO/R=2903; and H30: EaCO2/R=3637 and EaCO/R=2843. These results indicate differences in the structure of combustion reaction chains involved by the SAF addition to Jet A-1 fuel. The same conclusion has been formulated on the basis of the reactivity coefficient αi. The values of maximum cylinder pressure (Pmax) obtained during indicator RCCM (rapid compression combustion machine) tests correlated with both the activation energy and coefficients of reactivity. This suggests that the influence of SAF addition to Jet A-1 fuel on the structure of chemical reactions chain during RCCM tests is similar to the influence during DGEN 380 tests. The assumption stated above was confirmed. This indicates the possibility of the preliminary forecasting of CO2 and CO emissions from the DGEN 380 engine based on the test at the RCCM stand.