A theoretical model is presented which describes time dependent charging phenomena in insulators irradiated by an electron beam. This so called dynamic double layer model (DDM) is based on the simplified assumption of charge development in two axi‐symmetric cylindrical volumes at the insulator surface and at a certain depth below the surface, respectively. Important physical interaction mechanisms like the electrostatic interaction between the surface potential and emitted secondary electrons, the transport characteristics of internally generated secondary electrons in a positively charged insulator surface, as well as radiation induced electrical conductivity are included in the DDM model. Experimentally determined stationary and time dependent charging behaviour of Al2O3single crystals (sapphire) irradiated with an electron beam between 1 and 15 keV energy can be qualitatively explained by the DDM model. ©1995 American Institute of Physics.