We have studied the positron diffraction from the Si(0 0 1) 2 x 1 surfaces. In the diffraction patterns the fractional order spots are brightly visible. The vertical positions of the dimer are determined from the numerical analysis of the rocking curves based on the dynamical diffraction theory at room temperature and 130 K. The heights of the two atoms of the dimer are both 1.3 +/- 0.2 angstrom at room temperature and 1.3 +/- 0.2 angstrom and 1.1 +/- 0.2 angstrom at 130 K from the layer below the dimers. (c) 2004 Elsevier B.V. All rights reserved.