We describe ground-based measurements of ClO made at McMurdo Station, Antarctica, during September and October 1992. Vertical profiles were retrieved from molecular rotational emission spectra at 278 GHz. Peak mixing ratios of 1.6¿0.3 ppbv were seen in mid-September at approximately 18 km altitude, suggestive of somewhat larger quantities than were measured at the same site and season in 1987. As the core of the polar vortex moved away from McMurdo by early October, the ClO mixing ratio at this altitude dropped to less than 0.2 ppbv, coincident with increasing temperature, ozone, and NO2. The diurnal variation of ClO was also observed. The lower stratospheric layer from 15 to 27 km was found to reach approximately midday abundance by 2--3 hours after sunrise. The column abundance in this layer began to decrease by the period 4--2 hours before sunset and had declined to approximately one quater of its midday value by 2--0 hours before sunset. In contrast, the ClO column in the upper stratosphere, from 28 to 50 km, built up slowly until midday and remained large while sunlight persisted. ¿ American Geophysical Union 1995