The dilution effects of inert components N-2 and CO2 on the propagation and extinction of lean premixed H-2/CO/air syngas flames were experimentally and numerically investigated. Extinction stretch rates were measured using the counterflow technique while laminar flame speed data were obtained from literatures. Numerical simulations were conducted at 1-D freely propagating configuration and opposed-jet configuration with detailed chemistry and molecular transport description. The numerical results well predicted the experimental measurements. Both results revealed that CO2 dilution had more profound effect on flame propagation and extinction than N-2 dilution. In addition, numerical simulation assessed the preferential importance in a rather quantitative manner among the three effects, i.e., the thermal effect, the diffusivity change effect and the chemical effect with artificial manipulation of mass diffusivities and chemical reactions of CO2 and N-2. The results showed that the thermal effect dominated the reduction of laminar flame speed and extinction strain rate. The chemical effect caused by CO2 dilution was slightly stronger to the reduction of extinction limit than to that of laminar flame speed. The diffusivity change effect is negligible for both CO2 and N-2 dilutions. N-2 only acts as a thermal inert in the propagation and extinction of the H-2/CO/air flames. (c) 2015 Elsevier Ltd. All rights reserved.