The ion composition of the upper atmosphere has been the subject of many experiments. Early mass spectrometer measurements have been primarily concerned with the ion composition at altitudes between 60 and 250 km (refs 1-6). Few data, however, are available on the charged particle composition of the stratosphere. Global ion density measurements using electric probes 7-11 indicate number densities of the order of 100-10,000 cm -3 for positive ions in this region. Arnold et al.12 have recently published the first mass spectrometric data of the stratospheric ion composition, obtained on the downleg portion of three rocket flights. Above 40 km they observe the proton hydrates as being the most abundant ions. Below 40 km a change in the predominant ion species is seen, which is explained by Arnold et al. by ion molecule reactions between water cluster ions and formaldehyde. One of the possible dangers, inherent to rocket flights, however, is the alteration of the ion composition, due to shock wave-induced fragmentation. Mass spectrometric measurements on this region using a balloon-borne instrument were suggested13 as early as 1969. Here we give the first preliminary results of a successful flight, performed with a balloon-borne quadrupole mass spectrometer on 30 September 1977. A 100,000 m3 Zodiac balloon was flown at mid-latitude (CNES lauching site at Aire sur l'Adour, France, 44 °N) at 18.27 UT and reached the altitude of 35 km at 20.00 UT. The measurements were started at 20.14 UT, which is after sunset.