In situ electron microscope observations of defects show that the incident high-energy electrons influence the evolution of microstructure of an irradiated material, reducing the number of defects seen in the field of view of the microscope. We investigate the origin of this phenomenon, using tungsten as a case study. We find that displacements of atoms due to electron impacts give rise to the stochastic jumps of a defect over several interatomic distances, but the frequency of these events under normal conditions is too low to influence the high thermal mobility of defects in a pure material. At the same time our analysis shows that migration of defects driven by electron impacts provides the dominant mechanism of diffusion in a material where the defects are pinned by solute atoms or impurities. (c) 2006 Elsevier B.V. All rights reserved.