Ionization damage (radiolysis) and knock-on displacement are compared in terms of scattering cross section and stopping power, for thin organic specimens exposed to the electrons in a TEM. Based on stopping power, which includes secondary processes, radiolysis is found to be predominant for all incident energies (10–300 keV), even in materials containing hydrogen. For conducting inorganic specimens, knock-on displacement is the only damage mechanism but an electron dose exceeding 1000 C cm−2 is usually required. Ways of experimentally determining the damage mechanism (with a view to minimizing damage) are discussed. Microsc. Res. Tech., 2012. © 2012 Wiley Periodicals, Inc.