Abstract The application of HREM to the study of the mechanism of solid state chemical reactions looms as one of greatest importance to chemistry. Time-resolved studies to observe the progress of slow reactions can be made by successive application of well developed conventional procedures with or without treatment between observations. If the reactions occur over seconds or minutes, it is necessary to utilize techniques currently being developed for recording high-resolution images in real time either on video tape or digitally in the computer. These dynamically recorded images must then be further processed and analyzed. A brief survey of recent, relevant literature will be made as an introduction to studies currently being carried out in our laboratories. We have chosen to study chemical reactions that occur in thin films of one element as another diffuses into it. Specifically, juxtaposed members of diffusion couples consisting of Au-Sn and Au-In have been examined utilizing the sequential application of conventional methods of imaging and analysis. The results include the observation of compounds expected from the phase diagram as well as the formation of previously unreported phases. These new substances may be metastable or their stability credited to the thin-film configuration. It is now possible to make real-time observations of chemical reactions on a TV monitor with simultaneous recording on video tape. For example, the deposition of Au islands on thin-film grains of In resulted in the formation of Au 3In, and a record of this reaction was preserved on video tape for replay or further processing. The processing techniques to accent the essential features of the reaction will be discussed as well as the results of this and other studies.