Phlogopite micas have been studied at high temperatures in the electron microscope and the observations related to the known behavior of micas to dehydrate, to undergo swelling over a wide temperature range, and to their anomalous thermal expansion. The specimens showed evolution of bubbles at room temperature and also on heating in the electron microscope. The bubbles are lenticular flaws from the separation of (0001) cleavage planes and these lenticular flaws exhibit intricate Bragg extinction contours. Areas of lenticular flaws frequently display complex diffraction patterns. The bubbles occur particularly on dislocation networks, tend to nucleate and grow at nodal points, and whole dislocation networks have been observed to convert at 700°C into large lenticular flaws It is shown that dehydration of mica can refer to two different processes. The first below 500°C is the removal of water molecules from between the silicate layers into lenticular flaws and the second above about 800°C is the escape of water from the mica by splitting of the crystal.