The atomic structure of the Sb-terminated InSb(001)-c(4 4) reconstruction has been determined using X-ray diffraction. A total of 132 in-plane reflections were used to determine the structure in the surface plane. The out-of-plane structure was calculated by measuring intensity profiles along four integer-order and two fractional-order diffraction rods. The primitive unit cell of the c(4 4) phase consists of a group of three symmetric Sb dimers chemisorbed onto a complete Sb layer. The groups are found to be incomplete in ∼ 1/3 of the cases. The calculated bond length of the central dimer is 2.91±0.02 . The outer dimers are extended with a separation of 3.14±0.02 . The bond angles are indicative of sp 3 -type bonding. The data suggests that the reconstruction is limited mainly to the top layer, with only slight relaxation of the underlying bulk layers.