We have performed measurements of stratospheric chlorine monoxide (ClO) in Ny-¿lesund, Spitsbergen (78.9 ¿N, 11.9 ¿E), using a ground-based mm-wave radiometer. In this paper we describe the observed degree of chlorine activation inside the polar vortex in the winter 1996/97. We obtained daily averaged ClO mixing ratios for the lower stratospheric layer on 20 days covering the period from mid-February until the beginning of April. The volume mixing ratio of the lower ClO peak at an altitude of approximately 21 km reached a maximum of 1.6 ppbv. These measurements support the strong chemically induced ozone depletion which was found until April 5 by simultaneous ozone measurements reported in a companion paper [Sinnhuber et al., 1998>. They observed an unusual ozone loss in late April, which corresponds to the observed ClO abundance on April 18--22. On several days we observed diurnal ClO cycles which were compared with model calculations. ¿ 1998 American Geophysical Union