The electron-beam induced damage in N-rich SiNx:H films, deposited on Si by plasma enhanced chemical vapour deposition, is studied using cross-section transmission electron microscopy. It is demonstrated that these films are unstable under electron beam irradiation, during conventional transmission electron microscopy observations (120 keV), which causes strong outdiffusion of Si from the substrate into the N-rich layer. This phenomenon leads in a number of changes which include the evolution of voids at the SiN/Si interface and the development of morphology in the previously featureless (as-grown) films. Finally, plane-view electron diffraction patterns identify the presence of unreacted Si embedded in the matrix of the amorphous SiN,