Fusion-bonded silicon wafers exhibit a superstructure at their common interface due to the spatial beating of the two crystal lattices. The superstructure consists of a network of screw dislocations with a period determined by the twist angle theta. By synchrotron X-ray diffraction, the periodic elastic modulation in the two crystals resulting from the dislocation network has been measured. The characteristic thickness of the modulated region is found to be inversely proportional to theta, reaching over 160 Angstrom for theta = 0.4 degrees. This behavior is reproduced in numerical simulations of the elastic modulation. (C) 1999 Elsevier Science B.V. All rights reserved.