A photoemission and LEED–Auger study has been made to investigate the effect of oxygen adsorption on the electronic and atomic structure of a reconstructed Ir(100) surface. The clean surface is found to have a 5d-band width of 8.0 eV. Origin of an intrinsic surface state and other possible effects of the reconstruction are discussed. Upon adsorption of O2 an oxygen 2p peak emerges 11.1 eV below the vacuum level. The influence of the electronegativity difference between oxygen and the substrate metal on the photoemission is also considered.