In recent times stationary gas engines, especially those fuelled with poor gases such as biogas or landfill gas, have shown amounts of formaldehyde (HCHO) emissions exceeding the given limits of the German emission regulation TA Luft. In order to achieve compliance with the emission regulations, research was conducted at the Lehrstuhl fur Verbrennungskraftmaschinen of the Technische Universitat Munchen to discover the source of formaldehyde emissions as well as determining the influence of internal engine modifications on emissions. This research project No. 918 was initiated by the Forschungsvereinigung Verbrennungskraftmaschinen e. V. (FVV, Research Association for Combustion Engines). For investigations a 4-l and a 1.7-l single-cylinder SI engine were used.