Although a large number of measurements of inner potential have been made in the past, the results have, on the whole, been far from consistent. These measurements were made by electron diffraction methods, and one possible reason for the lack of consistency is that, owing to the very limited penetration of the electron beam, the nature of the surface layers of the specimen will play an important part in the results obtained. In particular, if the specimen is a single crystal, there arises the possibility that different results will be obtained if the measurements are made using crystallographically different surfaces of the crystal. This possibility was examined experimentally and, for the materials used, no difference was found in the results obtained from different surfaces of the same crystal. Measurements were also made, using fast electrons (60 kV), of the inner potentials of metal crystals, and it was found that consistent results could be obtained in all cases if suitable experimental precautions were observed. It was also found that the presence of a thin contaminating layer on the surface (e.g. an oxide layer) did not affect the results provided the electron beam was able to penetrate the first few atomic layers of the specimen itself.