Downstream interactions between lean premixed flames with mutually different fuels of (50% H-2 50% CO) and CH4 are numerically investigated particularly on and near lean extinction limits in order to provide fundamental database for the design of cofiring burners with hydrocarbon and syngas under a retrofit concept. In the current study the anomalous combination of lean premixed flames is provided such that even a weaker CH4-air flame temperature is higher than a stronger syngas-air flame temperature, and, based on a deficient reactant concept, the effective Lewis numbers Le(eff) approximate to 1 for lean premixed (50% H-2 + 50% CO)-air mixture and Le(D) < 1 for CH4-air mixture. It is found that the interaction characteristics between lean premixed (50% H-2 + 50% CO)-air and CH4-air flames are quite different from those between the same hydrocarbon flames. The lean extinction boundaries are of slanted shape, thereby indicating strong interactions. The upper extinction boundaries have negative flame speeds while the lower extinction boundaries have both negative and positive flame speeds. The results also show that the flame interaction characteristics do not follow the general tendency of Lewis number, which has been well described in interactions between the same hydrocarbon flames, but have the strong dependency of direct interaction factors such as flame temperature, the distance between two flames, and radical-sharing. Importance of chain carrier radicals such as H is also addressed in the downstream interactions between lean premixed (50% H-2 + 50% CO)-air and CH4-air flames. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.