Commercially available LEED equipment permits a relative accuracy of the order of a few per cent, in the absolute measurement of the lattice identity period parallel to the surface. A technique is described by which the accuracy is increased to 0.5 per cent. Precautions necessary for control of the residual fields and the geometry of the diffraction system are listed. The manipulation of the specimen is optically monitored and a built-in reference ensures the accuracy of the diffractogram evaluation on the polaroid film. The surface lattice constants are calculated by linear regression analysis. The technique has been applied to investigate the (111)-surface of silicon. The measured distance between atomic rows in the (111) plane amounted to 3.366± 0.012 Å which deviates significantly from the bulk value of 3.326 Å.