The chemical and dilution effects of the major exhaust gas recirculation constituents N-2, CO2 and H2O on the reactivity of iso-octane/air and n-heptane/air mixtures were investigated by separated weak flames in a micro flow reactor with a controlled temperature profile. Experimental results showed that high dilution ratios of 20% and 50% of N-2 and CO2 shifted the blue flame (reactions of intermediate species) as well as the hot flame (reactions of the hydrogen/oxygen-system and CO) to higher wall temperatures, indicating a decrease of reactivity. The cool flame at low temperatures was not affected by these dilutions. On the other hand, dilution by H2O showed a promoting effect on the reactivity as it shifted the blue flame and the hot flame to lower wall temperatures. In 1-D steady computations by the modified KUCRS mechanism, good agreement was found with the experiment except for dilution by H2O. For the comparison of flame locations, a new method was employed that calculated the photon emission from excited CH* and CO2*. While the strong effect of H2O on the reactivity was not observed in the simulation, it was attributed to its high enhanced third body efficiency, which is important for three body chain branching and chain termination reactions of the hydrogen-oxygen system. The contribution of each separated weak flame to the total heat release was calculated by the heat contribution index. This index revealed a stronger effect of the three diluents on n-heptane than on iso-octane. This effect was found to be corresponding to an increase of the research octane number by 28 for the addition of 50% H2O to the n-heptane/air mixture. For iso-octane, the corresponding increase of the RON was 12. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.