Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H(2) + 70% N(2). Extinction limits are determined for methane/air and (30% H(2) + 70% N(2))/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures' concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.