Acetone was measured in the free troposphere and lower stratosphere over the North Atlantic by aircraft-based CIMS (Chemical Ionization Mass Spectrometry) and GC (Gas Chromatography) instruments in fall 1997 within the framework of the POLINAT-2/SONEX campaigns. The measurements included 28 flights covering altitudes from 2.4 to 12.5 km, and geographic latitudes from 20 to 69 ¿N. The data are the most extensive measurements of acetone in the free troposphere and lower stratosphere published to date. A first comparison of the CIMS and the GC method is presented. The acetone data obtained by both instruments in the same air masses agree within 50% which is consistent with the uncertainty limits quoted. An inspection of all flights reveals upper tropospheric acetone to vary very substantially between 200 and 2200 pptv with a mean value of about 600 pptv. By contrast, in the lower stratosphere the acetone abundance is much less variable and is mostly around 100 to 200 pptv. Acetone rich air masses with volume mixing ratios exceeding 1000 pptv and reaching up to 2200 pptv were surprisingly frequently encountered in the upper troposphere. Probably these air masses had recent contact with the continental planetary boundary layer where they took up acetone and/or acetone precursor gases from ground-level sources. ¿ 1999 American Geophysical Union