The structure of the GaAs(001)-2 4 reconstructed surface is studied using medium-energy ion scattering (MEIS). The MEIS profiles clearly show splitting blocking dips, which originate in the surface reconstruction. We have quantitatively determined the atomic displacement from these blocking dips. The topmost As atoms forming the symmetric dimers are displaced laterally 0.55 ±0.05 , and the bond length of the As dimer is 2.89 ±0.1 . The Ga atoms of the second atomic layer, which bond with the As dimer atoms, are also displaced laterally and the displacement is 0.44±0.05 . These surface atoms also move vertically and the surface layers shrink.