Abstract The emission indices of aircraft engine exhausts from measurements taken under operating conditions, to calculate precisely the emission inventories of airports, are not available up to now. To determine these data, measurement campaigns were performed on idling aircraft at major European airports using non-intrusive spectroscopic methods like Fourier transform infrared spectrometry and differential optical absorption spectroscopy. Emission indices for CO and NOx were calculated and compared to the values given in the International Civil Aviation Organisation (ICAO) database. The emission index for CO for 36 different aircraft engine types and for NOx (24 different engine types) were determined. It was shown that for idling aircraft, CO emissions are underestimated using the ICAO database. The emission indices for NOx determined in this work are lower than given in the ICAO database. In addition, a high variance of emission indices in each aircraft family and from engine to engine of the same engine type was found. During the same measurement campaigns, the emission indices for CO and NO of eight different types of auxilliary power units were investigated.