The alignment of the conduction and valence bands in metal/mixed Hf-Ta oxide/silicon structures was determined as a function of oxide composition using intrinsic photoconductivity and internal photoemission measurements. With increasing Ta content in the Hf-Ta oxide from 0% to 100%, the band gap gradually decreases from 5.6 to 4.2-4.4 eV. This is predominantly associated with the downshift of the lowest conduction band derived from the 5d(*) unoccupied orbitals of the metal cations. This indicates a significant mixing of the states belonging to the Ta and Hf subnetworks, and suggests the possibility of oxide band gap control in the mixed oxides of metals from the same period of elements. (C) 2005 American Institute of Physics.