High carbon monoxide (CO) emission from gas engines fueled by producer gas is a concerning problem in the struggle to make biomass gasification for heat and power production a success. CO emissions from engines operating on biomass producer gases are high, especially at very lean conditions where the emissions exceed the regulated limit significantly. The high CO emissions are mainly due to the high content of CO in the fuel and can ¿ in origin ¿ be compared with the emission of unburned hydrocarbons (UHC) from natural gas engines, thus CO emissions from producer gas engines are a measure of fuel passing unburned through the combustion. Measurements of the slip of the producer gas fuel components CO and CH4 showed that these are of similar order. When the environmental effect of the emissions is discussed, unburned hydrocarbons in the form of methane is a strong greenhouse gas (21 times higher than CO2) while CO only indirectly through photochemical reactions is involved in the production of the greenhouse gas ozone.