SummaryIn this paper an electron diffraction method is discussed by which the lattice constants of polycrystalline thin films can be accurately determined (0.1%).The method involves the sequential examination of a standard material and the unknown material mounted on separate grids. The error which can arise through the possible difference in height of the two grids can be corrected by means of height and tilting adjustments. The analytical approach of the dependence of the camera constant on the ring diameter as given in literature appears to be insufficient. An experimental correction factor for this dependence is introduced. The accuracy of this method, and the influence of relevant sources of error, are discussed and explained in terms of some experimental results.