Cobalt oxide films were prepared by oxidation of different amounts of cobalt deposited onIr(100)-(1 × 1), where oxygen rich conditions were applied during deposition. The resulting oxide films withthicknesses of up to about 40 Å were investigated as regards their crystallographic structureand morphology, applying quantitative low energy electron diffraction (LEED) andscanning tunnelling microscopy (STM). It can be unequivocally shown that the spinel-typeCo3O4 phasedevelops, for which an excellent fit between measured and calculated LEED intensity spectra is achieved(Pendry R-factor R = 0.124). In spite of the quadratic unit cell of the substrate the oxide films are in the polar (111)orientation. Also, the native lattice parameter of the material is assumed, i.e. there is nopseudomorphic relation to the substrate. However, by means of orientational epitaxy, one ofthe unit-mesh vectors of the oxide and one of those of the substrate layer are aligned,leading to two mutually orthogonal domains in the oxide. The oxide is terminatedby a sublayer of cobalt ions which in the bulk were tetrahedrally coordinatedCo2+ ions. There are drastic relaxations of layer spacings at and near the surface. As aconsequence, the bond length between the surface terminating cobalt ions and oxygen ionsbelow is considerably reduced, indicative of a substantial change of the ionicity of thecobalt and/or oxygen ions. This is interpreted as accounting for polarity compensation ofthe film, as surface reconstruction, oxygen vacancies and species adsorbed can be ruled out.