A simple method for the relative measurement of the interplanar distances of high perfect crystals with a high precision is described. It is based on the comparison of the interplanar distances of two crystals using neutron transmission through one crystal and back reflection by the reference second crystal at Bragg angles close to 90{ring operator}. The method allows us to measure the variation of the crystal interplanar distance Δ d relatively to the reference with the accuracy Δ d / d better than 10- 7. © 2008 Elsevier B.V. All rights reserved.