Results from a series of 12 ER-2 aircraft flights into the Antarctic polar vortex are summarized. These in situ data define the spatial and temporal distribution of ClO as the aircraft flew at an altitude of ~18 km from Punta Arenas (54 ¿S latitude, 70 ¿W longitude) to the base of the Palmer Peninsula (72 ¿S latitude, 75 ¿W longitude) and back. At the southern extremity of the flight trajectory theaircraft descended to ~13 km, turned north, and ascended to 18 km. A general pattern in the ClO distribution is reported; mixing ratios of approximately 10 parts per trillion by volume (pptv) are found at cruise altitude in the vicinity of 55 ¿S increasing to 50 pptv at 60 ¿S. In the vicinity of 65 ¿S latitude a steep gradient in the ClO mixing ratio is observed. At a fixed potential temperature, the ClO mixing ratio increases by an order of magntitude within a very few degrees of latitude through this sharp transition, thus defining the edge of a ''chemically perturbed region.'' From the edge of that region to the southern extension of the flights, 72 ¿S, a dome of slowly increasing ClO best describes the distribution. Peak mixing ratios at 18 km at the southern extension of the flight track increased from 800 pptv on August 23 to 1100 pptv in early September and then increased slowly to 1200 pptv by the end of the mission on September 22. ¿ American Geophysical Union 1989