Spatially and spectrally resolved in-cylinder absorption measurements were performed in spark-ignited internal combustion engines and in Diesel engines. With UV-broadband illumination it was shown that the UV attenuation occurs throughout the burned gas area with roughly homogeneous absorption cross-sections. Model calcns. based on the absorption properties of CO2 at elevated temps. show that this species gives the main contribution to in-cylinder UV absorption. A previously suggested technique of assessing UV absorption using O2 laser-induced fluorescence (LIF) as probe light is successfully applied to in-cylinder measurements of the light absorption inside a fired heavy-duty Diesel engine. Even in this environment, the comparison with model calcns. shows that CO2 is the main contributor to UV light absorption. Since the O2-LIF absorption technique is based on the identical geometry used for LIF concn. measurements, the results can directly be used for correcting LIF signal data such as that obtained from NO imaging.