In order to determine the origin of the CO emissions from a cogeneration spark ignition engine fuelled with a lean natural gas-air mixture, the influence of several combustion chamber parameters have been measured in detail. In an attempt to interpret the experimental results, the engine process was simulated to dissociate the contribution of the bulk gas combustion kinetics from the crevice hydrocarbon burn-up or other sources. Modelling of CO kinetics in the bulk gas leads to the conclusion that although CO kinetics result in above equilibrium values of CO at exhaust valve opening, these values account for only approximately 5 to 10% of the concentrations measured in the exhaust gas.