Autoignition for most fuel-air mixtures in engines is preceded by relatively slow chemical changes. These changes are the main area of interest in this contribution, since a better control of the autoignition process in homogeneously charged motored engines may become potentially more viable through a better understanding of the reactions that lead to autoignition. An approach to modify the partial oxidation process is by changing the composition of the charge through a deliberate recirculation of some of the exhaust gases back into the cylinder. These recycled gases, when not fully cooled, can influence the autoignition process thermally. They may also contain small concentrations of active chemical species that could influence kinetically the partial oxidation processes of the engine.The paper describes the results of an analytical approach that models the reaction rate development in a naturally aspirated, motored, n-heptane fuelled, homogeneously charged engine when recycled exhaust gases are applied in a controlled manner. A kinetic scheme of 683 reaction steps and 158 species was used for this simulation.The effects of the deliberate seeding of the fresh charge by certain selected components of the recycled exhaust gases are also examined and the species most active kinetically in bringing about autoignition identified.