A campaign utilizing an ER-2 high-altitude aircraft and a DC-8 aircraft, both fitted with state-of-the-art instrumentation to study the Antarctic ozone hole, was conducted out of Punta Arenas, Chile, from August 17 through September 22, 1987. Data indicated a chemically perturbed region roughly coincident with the Antarctic polar vortex and with the region of large temporal decrease of ozone that is usually referred to as the Antarctic ozone hole. A rapid rise in ClO was observed as the ER-2 proceeded into the ozone hole at about 18 km altitude, and it is this feature that is used to define the boundary of the chemically perturbed region as that latitude along the flight track where ClO reaches 130 parts per trillion by volume (pptv). In situ data taken simultaneously aboard the ER-2, as well as Total Ozone Mapping Spectrometer (TOMS) satellite ozone data along the flight tracks, are analyzed at fixed positions relative to this boundary and are presented as average over the duration of the mission. These analyses indicate a narrow transition zone at the boundary for the chemically active species ClO, O3, NOy, and NO. A somewhat wider transition zone for the chemical species N2O and H2O and for the meteorological parameters of temperature, wind speed, and potential vorticity is also seen, indicating the dynamical character of the chemically defined boundary. TOMS column values of about 260 Dobson units (DU) generally persisted at the boundary during this period. One-month temporal trends of the in situ data both inside and outside this boundary are also presented. Interpretations of these analyses are offered that are consistent with ongoing diabatic cooling, accompanying advective poleward transport across the boundary. These data strongly implicate man's release of chlorine into the atmosphere as a necessary ingredient in the formation of the Antarctic ozone hole. ¿ American Geophysical Union 1989