The structure of two GaSb layers, 13 nm and 327 nm thick, grown by metal-organic vapour-phase epitaxy on a GaAs(001) substrate have been studied by means of high-resolution x-ray diffraction. The large lattice mismatch is largely relaxed by regular arrays of 90° dislocations. The results show that both layers have orthorhombic crystal structure, and provide detailed quantitative information about the distortions caused by the dislocations in both the GaSb layers and the GaAs substrate.