Plume expansion from nine space shuttle and Titan IV vehicles was measured at altitudes of 18, 24, and 30 km in the stratosphere. The plume diameters, inferred from electronic images of polarized, near-infrared solar radiation scattered from the exhaust particles, increased linearly with time. The expansion rate was measured for as long as 50 min. Observations made simultaneously at multiple altitudes showed that the expansion rate increased with increasing altitude for six measurements made at Cape Canaveral but decreased between 24 and 30 km for the one measurement made at Vandenberg Air Force Base. The average expansion rates for all measurements are 4.3¿1.0 m s-1 at 18 km, 6.8¿1.9 m s-1 at 24 km, and 8.7¿2.4 m s-1 at 30 km. Expansion rates varied from launch to launch by as much as a factor of 1.6 at 18 km, 2.2 at 24 km, and 2.6 at 30 km. No correlation between the expansion rate and wind speed or wind shear was evident. These data are compared with several models for diffusivity and are used to update a comprehensive particle model of solid rocket motor exhaust in the stratosphere. The expansion rates are required by models to calculate the spatial extent and temporal persistence of the local stratospheric ozone depletion cause by solid rocket exhaust. ¿ 2000 American Geophysical Union