In situ O3 and ClO data obtained from the ER-2 aircraft are used to define the chemical evolution of the Antarctic vortex region from August 23 to September 22, 1987. Initial conditions are characterized at aircraft flight latitude (18 km) by highly amplified ClO mixing ratios (800 parts per trillion by volume (pptv)) within a well-defined ''chemically perturbed region'' (CPR) poleward of the circumpolar jet, within which ozone exhibits limited erosion (~15%) in middle to late August. Within this CPR, ozone decays consistently throughout the course of a 10-flight series, such that by late September, 75% of the O3 has disappeared within the region of highly amplified ClO concentrations (which reached 500 times normal levels at ER-2 cruise altitude). As this ozone depletion develops, O3 and ClO exhibit dramatic negative correlation on isentropic surfaces, obtained as the aircraft passed through the edge of the CPR. Taken in conjunction with an analysis of the mechanisms defining the rate of catalytic O3 destruction, it is concluded that ClO is an essential constituent in the catalytic destruction of ozone within the vortex. Therefore it is concluded that the observed disappearance of ozone within the Antarctic vortex would not have occurred in the absence of global chlorofluorocarbon release. ¿ American Geophysical Union 1989