A customized Fourier transform infrared spectrometer on board an aircraft performs in-flight measurements of the jet engine's exhaust emissions. For this purpose the instrument is mounted in the aircraft's cabin receiving infrared radiation emitted by the hot jet about 1 m downstream from the nozzle exit. From the measured infrared spectra the concentrations and emission indices of the gas species NO, CO, H2O, and CO2 and the gas temperature are calculated. The estimated accuracy of the measurements is in the range of 5 to 20%. The results reveal the high precision of the acquired data which are compared to model calculations. ¿ American Geophysical Union 1996