From 30 August 1992 to 3 September 1992 a supersaturated area at 465 K potential temperature (~50 hPa) is deduced from MLS water vapor measurements over western Antarctica, where high extinction coefficients measured by CLAES indicate Polar Stratospheric Clouds (PSCs). These PSCs are attributed partly to the effect of an anticyclone located over South America and partly to localized orographic waves, which raise the isentropes and generate rapid adiabatic cooling. A local minimum in column O3 (≤200 DU) is observed in this area, which is believed to be a consequence of the dynamics. Enhanced ClO abundances downstream of the region indicate PSC processing and chlorine activation. ¿ American Geophysical Union 1995