Measurements by the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) are being used to study the spatial and temporal evolution of the volcanic stratospheric aerosol from Mount Pinatubo. The maximum opacity of the aerosol cloud moved from a position south of the Equator at an altitude of about 26 km in early October 1991, became located over the Equator by mid-January 1992, and descended in altitude to about 21 km by July 1992. Dispersal of the cloud was more rapid in the Southern Hemisphere and penetration to the southern polar region occurred earlier than transport to the corresponding northern polar area. The area weighted global mean stratospheric optical thickness between 15 km and 35 km at 12.1 &mgr;m remained at about 5.5¿10-3 from November 1991 through to April 1992. The estimated aerosol mass loading is 19--26 megatonnes for this period and by the end of July 1992 it had declined to 15--21 megatonnes. ¿ American Geophysical Union 1993