MLS H2O, CLAES N2O, and potential vorticity calculated from UK Meteorological Office data are used to study mid-stratospheric vortex processes in the northern hemisphere winter of 1991--92. Areas of moist air (at ~20 hPa) and N2O-poor air (at ~10 hPa) are well-correlated with high values of potential vorticity and there is little or no large scale mixing across the vortex edge. We find evidence for the descent of relatively dry mesospheric air to the 840 K (~10 hPa) level, as well as descent of moist air from the upper stratosphere to the 655 K (~20 hPa) level. A reduction in the areas of the vortex and both the moist and N2O-poor regions is observed and there is evidence of moist and N2O-poor air parcels being extruded from the vortex. ¿ American Geophysical Union 1993