Abstract Laminar flame speeds of lean premixed H 2 /CO/air mixtures were measured in the counterflow configuration over a wide range of H 2 content at lean conditions. The values were determined by extrapolating the referenced flame speed to zero stretch rate using the non-linear extrapolation method to reduce the systematic error. Detailed calculation of laminar flame speed was also conducted using PREMIX code coupled with three different kinetic models. In general, simulation results agreed well with the experimental data. Both the experimental and calculation results revealed that the laminar flame speeds of lean premixed H 2 /CO/air mixtures increased with H 2 content significantly when H 2 content was small (⩽15%) and gradually when H 2 content was large (>15%).