Abstract. Upper tropospheric and lower stratospheric acetone measurements have been performed in summer and winter 1994 through 1996 at latitudes between 30øN and 75øN using ionmolecule reaction mass spectrometry. We observed very high acetone volume mixing ratios of up to 3000 pptv (parts per trillion by volume) in extended air masses and in summer when acetone destruction by photodissociation is fast. This indicates efficient transport of acetone and photochemical acetone precursors to the upper troposphere and efficient upper tropospheric formation of acetone products, especially HO x radicals and PAN. Our data indicate large HOx production from acetone which has important implications for other trace gases and aerosols.