During the European Arctic Stratospheric Ozone Experiment (EASOE) in winter 91/92, first measurements with a new 204 GHz radiometer were performed at the Jungfraujoch Alpine Station (7¿59'E 46¿33'N) in the Swiss Alps. The altitude of this site (3570 m) allows detection of weak spectral lines like the ones of the chlorine monoxide J 11/2 → 9/2 rotational transition. At the end of January 1992, a large amount of ClO in the lower stratosphere was measured, indicating that highly perturbed airmasses had been transported towards mid latitudes. The maximum value of the ClO volume mixing ratio was about 1 ppbv +/-0.4 ppbv in heights of 15--25 km. ¿ American Geophysical Union 1994