The effects of CO2 addition on the downstream interactions of H-2-air and CO-air premixed flames were investigated. Flame stability diagrams mapping the extinction limits for various fuel concentrations as functions of the strain rate were developed for the downstream interactions between H-2-air and CO-air premixed flames with 0-20% CO2 addition. Of particular concern was the impact of the chemical reactions of the additional CO2 on the flame extinction characteristics. The results showed that CO2 addition suppressed the flammable regions appreciably and decreased the critical strain rate, opening up the lean extinction boundary. For highly stretched interacting flames, two flammable island regions on the maps were combined into one when 10% or 20% CO2 was added to either the H-2-air or CO-air premixed mixtures. The chemical reactions of the additional CO2 were significant, especially for high strain rate flames, while the effect of the radiative heat loss of the additional CO2 was minor, even for low strain rate flames. Such chemical reactions were determined by competition between the reaction steps for the consumption and production of HCO and the reverse reaction of CO + OH = CO2 + H with the reaction step of H + O-2 -> O + OH, an indicator of the overall reaction rate for the H atoms. This resulted in a weakened interacting flame. Such chemical reactions of the additional CO2 were examined in detail to describe the flame characteristics. (C) 2014 Elsevier Ltd. All rights reserved.