Laminar burning velocity measurements of 50:50 and 85:15% (by volume) H2–CO mixtures with O2–N2 and O2–He oxidizers were performed at lean conditions (equivalence ratio from 0.5 to 1) and elevated pressures (1 atm–9 atm). The heat flux method (HFM) is employed for determining the laminar burning velocity of the fuel–oxidizer mixtures. HFM creates a one-dimensional adiabatic stretchless flame which is an important prerequisite in defining the laminar burning velocity. This technique is based on balancing the heat loss from the flame to the burner with heat gain to the unburnt gas mixture, in a very simple way, such that no net heat loss to the burner is obtained. Instabilities are observed in lean H2–CO flames with nitrogen as the bath gas for pressures above 4 atm. Stable flames are obtained with helium as the bath gas for the entire pressure range. With the aim to cater stringent conditions for combustion systems such as gas turbines, an updated H2–CO kinetic mechanism is proposed and validated against experimental results. The scheme was updated with recent rate constants proposed in literature to suit both atmospheric and elevated pressures. The proposed kinetic model agrees with new experimental results. At conditions of high pressure and lean combustion, reactions H + O2 = OH + O and H + O2 (+M) = H2 (+M) compete the most when compared to other reactions. Reaction H + HO2 = OH + OH contributes to OH production, however, less at high-pressure conditions. At higher CO concentrations and leaner mixtures an important role of reaction CO + OH = CO2 + H is observed in the oxidation of CO.