The electronic structure of nanostructured Co adatoms grown by step decoration of a vicinal Au(1 1 1) surface has been investigated by angle-resolved photoemission spectroscopy (ARPES). With increased Co coverage, the intensity of An 5d valence bands decreases but their peak positions do not change, suggesting that the mixing between An 5d and Co 3d bands is negligibly weak. The coverage dependence of ARPES spectra near the Fermi level for Co was compared with bulk photoemission results. At the coverage of 1.28 ML Co, the observed Co 3d bands were identified as the exchange splittings between 3d bands with the similar energy value to the bulk. (C) 2004 Elsevier B.V. All rights reserved.