The band structure of n-type diamond samples co-doped with boron and sulfur was studied using ultraviolet photoelectron spectroscopy (UPS) and Mott-Schottky analysis. The results show that the Fermi energy of the co-doped diamond is about 0.8-1.9 eV above the Fermi level of boron-doped diamond. The electron affinities of the samples ranged from -0.7 to -1.4 eV as evidenced by UPS. These results provide support for the presence of an acceptor impurity band within the bandgap region of the co-doped diamond samples. (C) 2004 The Electrochemical Society.