A light and easily operated balloon-borne UV-visible spectrometer was designed for investigating ozone chemistry in the Arctic winter. The optical sonde was flown 10 times during EASOE in northern Scandinavia in winter 1991--92 to measure O3 and NO2, the results of which are presented in a companion paper. On two occasions, 13 and 16 January 1992, during the coldest period of the campaign, when PSCs were visible from the ground, OClO mixing ratios of 16¿2 and 19¿3 ppt (1 s.d.) respectively were detected around 20 km at sunset. These are the first OClO profiles to be reported. They yield integrated columns of 5.8 and 8.9 1012 mol./cm2 above 16 km and 17 km respectively. These values are consistent with ground-based and aircraft total column measurements in the Arctic. ¿ American Geophysical Union 1994