During the European Arctic Stratospheric Ozone Experiment (EASOE) in 1991/1992 there were only a few occasions when polar stratospheric clouds (PSCs) were observed. One of these was on 27 January 1992 when PSCs were detected by two balloonborne aerosol instruments released from Esrange near Kiruna, Sweden (68¿N). High concentrations of particles were found at altitudes between 19 and 24 km, in a series of many very thin layers. Particles as large as 5 &mgr;m were detected. The surface area concentration in the densest layers exceeded 100 &mgr;m2 cm-3, as estimated from retrieved size distributions. Estimates of the total mass of the particles suggest that ice must have been the main constituent, although the air temperature was 3 K or more above the frost point. The particles were probably formed during leewave cooling over the Norwegian mountains, some hours earlier, and some 300 km upstream of the measurements. A possible explanation for the slow evaporation observed is offered in part II of this paper (Peter et al., this issue). ¿ American Geophysical Union 1994