Atmospheric emission spectra have been measured at an altitude of about 3000 m in the wavelength range from 17.5 to 20 microns. The very high spectral resolution (up to 0.02 kayser) together with the high signal-to-noise ratio achieved allow assignment of the observed lines to CO2, N2O or H2O. Existing spectroscopic molecular data have been compared with the observations. No serious deviations from the expected line frequencies are found for CO2 and N2O, but important discrepancies appear for H2(O-16) transitions occurring at energy levels greater than 2000 kaysers. A few lines due to H2(O-17) and H2(O-18) have been identified, and deviations from their expected frequencies as large as 0.3 kayser have been measured.