Investigations on the NOx reduction for natural-gas-fueled spark-ignition engines with lean-burn techniques revealed that NO2 is less sensitive to reduction measures than NO. For NO concentrations over 1000 ppm, the NO2/NO ratio is generally less than 0.1. For NO concentrations below 50 ppm, the NO2/NO ratio begins to exceed 1.0. The literature on burner and flow-reactor tests explains this phenomenon via the oxidation of NO with hydroperoxyl radicals through reactions very sensitive to component concentrations. Since NO is the precursor of NO2, measures to further lower the NO production will ultimately result in a lower NO2 emission. This has been demonstrated experimentally with an engine.