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In this paper, the formaldehyde emissions from three different types of homogenous charge compression ignition (HCCI) engines are quantified for a range of fuels by means of Fourier Transform Infra Red (FTIR) spectroscopic analysis. The engines types are differentiated in the way the charge is prepared. The characterized engines are; the conventional port fuel injected one, a type that traps residuals by means of a Negative Valve Overlap (NVO) and finally a Direct Injected (DI) one. Fuels ranging from pure n-heptane to iso-octane via diesel, gasoline, PRF80, methanol and ethanol were characterized.Generally, the amount of formaldehyde found in the exhaust was decreasing with decreasing air/fuel ratio, advanced timing and increasing cycle temperature. It was found that increasing the source of formaldehyde i.e. the ratio of heat released in the cool-flame, brought on higher exhaust contents of formaldehyde. The application of a standard three-way catalyst completely removed formaldehyde from the exhaust stream. INTRODUCTIONOver the last decade, the HCCI engine has shown a strong potential of realizing high efficiency in combination with low emissions for a range of fuels. However, many issues still have to be addressed and in previous works, questions have been raised on the presence of hazardous substances for our health, such as formaldehyde. Formaldehyde is a carcinogenic Aldehyde that isn't regulated as a single specie emission. However, it is indirectly limited by the restriction of the total hydrocarbon content in the exhaust.