The characteristics of tracks produced by fission fragments have been studied by transmission electron microscopy in the following layer structure materials: molybdenite (MoS2), talc, clinochlore, jefferisite, and various micas including biotite, phlogopite, muscovite, and synthetic fluor‐phlogopite. The measured track widths showed little correlation with the decomposition temperatures and it was concluded that the track formation cannot be described by a simple thermal spike model. The absence of any new rings or spots in the electron diffraction pattern of a heavily irradiated sample led to the conclusion that the tracks themselves consisted of a disordered region and not of a definable new crystalline phase. In many of the materials the tracks faded during observation in the beam. Although this fading could be eliminated by using a cold stage, it was not caused simply by the thermal instability of tracks; some materials which showed rapid track fading retained tracks to higher temperatures in external annealing experiments than did those in which no fading occurred. Annealing experiments also showed that tracks served to nucleate the high temperature decomposition of natural micas in a manner analogous to that previously reported for synthetic mica. The results of experiments in which Al absorbers were placed between a fission source and mica samples were interpreted as indicating that only particles with masses ≳ 30 are capable of forming visible tracks. This conclusion is consistent with the negative results of other irradiations in which mica samples were bombarded with 4‐MeV alpha particles, 150‐MeV oxygen nuclei, and 3‐BeV protons. Experimental observations of the interaction of dislocations with tracks are also presented.