LIDAR data of stratospheric aerosol layers from the Mount Pinatubo eruption, taken on long range flights with a Transall aircraft, have been investigated to study the exchange of air-masses across the polar vortex boundary. From these measurements we found that the center region of the stratospheric polar vortex was free of volcanic aerosol above PT values of 485 K throughout the whole winter, with the exception of small isolated clouds appearing at the end of Jan. 92. Below 420 K the aerosol layers are very homogeneous on an isentropic surface and do not show much correlation with the vortex. ¿ American Geophysical Union 1994