A new electron diffraction device is described which is especially suited for the investigation of diffusion problems, phase transitions, and reactions of solids with gases or other solids. It is equipped with a continuously recording camera where a 4×10 in. photoplate passes underneath a narrow slit exposed to the diffraction cone. The resulting photograph shows parallel diffraction lines instead of the usual Debye‐Scherrer rings and gives a gapless picture of the occurring structural changes which are easy to evaluate. If the sample is heated according to a known temperature‐time function, the temperature‐structure dependence may be recorded, and by moving the sample synchronously with the plate through the electron beam, one may record the geometrical distribution of different structures in an inhomogeneous sample. The latter method is especially suited for a rapid investigation of unknown binary or multicomponent phase diagrams. The device includes the following special features: deposition of the sample containing up to 3 different components by evaporation inside the specimen chamber, an attachment for monitoring the sample reflectance during runs, a rotating shutter producing wedge‐shaped sample layers, and a heatable specimen holder for use in transmission as well as in reflection. Finally, a few examples are given showing some applications of the described electron diffraction unit.