The laminar flame speeds of atmospheric lean syngas/air flames were measured using the expanding spherical flame under isobaric conditions. Two methods, namely extrapolation and DNS-mapping, were utilized to determine the un-stretched laminar flame speed with quantified uncertainty from the experimentally measured instantaneous stretched flame speeds. Results from four extrapolation expressions show that the extrapolation uncertainty can exceed 5% for equivalence ratio of 0.5 and hydrogen mole fraction of 0.6. Two detailed chemical mechanisms, the Li-mech and the HP-mech, were used in the DNS mapping. It is found that the uncertainty of DNS-mapping stays small, typically less than 2% for all the experimental conditions. The relative merits of extrapolation and DNS-mapping in determining the laminar flame speeds were then compared. Sensitivity analysis was also conducted and results show that H and OH radicals play crucial roles in the combustion chemistry of H-2/CO flames. Furthermore, near linearity between the mass burning rate and the maximum mole fraction/concentration of the (H + OH) radicals was empirically observed for the equivalence ratios of 0.5-0.9, H-2 content of 30-70% (by volume) at different pressures (1.0-50.0 atm) and initial temperatures (298-448 K). (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.