Extensive computations were made to determine the flammability limits of opposed-jet H-2/ CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H-2/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s(-1) where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H-2/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO2, H2O, and N-2 to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H2O contributes to higher flame temperature, while the radiation effect of H2O and CO2 plays an important role in the flame extinction at low strain rates. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.