Abstract The mechanisms proposed for physical change or for solid state chemical reactions are usually couched in phenomenological terms partly because of the difficulty of obtaining detailed information on occurrences at the atomic level. Use of the high-resolution electron microscope could provide the required atomic-level information in those instances where the rate of change is suitable. Two cases where the transformation rates are near the lower time limits of present capability are used here as illustrations. First, the removal of a twin boundary separating a surface promontory from the main body of a TbO 2 crystal is followed and analyzed from sequences of video-field images recorded at 1/60 second. Sudden disorder of the promontory is followed within a few seconds by reconstructive, epitaxial, outward growth on the main body of the crystal. Second, to illustrate similar analyses of a chemical reaction, an episode in the transformation of a lead zirconate titanate (PZT) precursor gel toward the crystalline PZT is observed at intervals from video-fields selected during condensation and the loss of solvent and the accompanying crystallization process. Areas of condensation on the surface of the colloidal sphere are amorphous at first but begin fitful crystallization when the diameter reaches about 3 nm and continues wild reconstructions until the stable perovskite is formed.