The structure and energetics of the hexagonal reconstruction of Ir(100) have been determined with first principles density functional theory calculations based on the local-density approximation with the generalised-gradient correction. The results reproduced the experimentally determined surface buckling and show the presence of some lateral displacement of the reconstructed (1 x 5) phase with respect to the ideal hexagonal close packed structure. The (1 x 5) phase is found to be 0.06 eV/(1 x 1 area) more stable than the (1 x 1)phase. The reconstruction is analysed by examining surface bonding. (C) 1998 Elsevier Science B.V. All rights reserved.