The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H(2) and 50% CO is numerically investigated by varying the strain rate in the range of 5 similar to 500 s(-1). The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H2 and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.