Based on XRD data testifying that the M ions occupy substitutional sites, transmittance measurement are discussed in comparison to electronic structure calculations for M-doped Al2O3 with M = V, Mn, and Cr. The M 3d states are found approximatively 2 eV above the top of the host valence band. The fundamental band gap of Al2O3 is further reduced in the V and Mn cases due to a splitting of the narrow band at the Fermi energy. Nevertheless the measured transmittance in the visible range remains high in all three cases. (C) 2011 Elsevier B.V. All rights reserved.