A combined experimental/simulation approach has been used to characterize the underlying deformation mechanisms associated with stress-assisted grain growth in nanocrystalline Al. Strain rate sensitivity experiments on freestanding submicron thin films undergoing stress-assisted grain boundary migration have uncovered rate sensitivities up to two orders of magnitude larger than previously reported for microcrystalline Al. Molecular dynamics simulations have been used to illustrate that these high strain rate sensitivities coincide with those associated with grain boundary processes such as migration, sliding, and dislocation nucleation. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.