We present a structural investigation of the polar (111) surface of CoO single crystals by grazing incidence X-ray diffraction. The surface is found to be always covered by a similar to 50 Angstrom-thick spinel Co3O4 layer that was quantitatively characterized. Attempts to remove this layer by UHV annealings or Ar+ bombardments lead to the formation of metallic Co clusters on the surface that transform again in Co3O4 by annealing under oxygen.