A deep understanding of the ignition characteristics of syngas in O-2/CO2 and O-2/H2O atmospheres is essential for the application of oxy-syngas combustion. In the present work, ignition properties of a syngas with a typical H-2-to-CO ratio (1:2) in O-2/N-2,O-2/CO2 and O-2/H2O atmospheres were investigated numerically. The ignition temperatures were determined by a 1-D model of a micro flow reactor with a controlled wall temperature profile, demonstrating that CO2 and H2O can lead to an increase in the ignition temperature compared to N-2, and the increase is more pronounced for the O-2/H2O atmosphere. The analysis manifests that CO2 and H2O can suppress OH production at the region with relatively lower wall temperature by promoting R10: H + O-2(+M) = HO2+(M) to compete with R11: H + HO2 = 2OH for H radical. Moreover, the direct reaction effect (directly take part in reactions as reactants) and third-body effect of CO2 and H2O on ignition temperature were numerically isolated by adopting artificial species. The computation results reveal that the increase in ignition temperature mainly results from the enhanced reaction rate of R10 by the third-body effects of CO2 and H2O. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.