Electron energy band alignment in GaAs(100)/Ga2O3/GdxGa0.4-xO0.6 structures was determined using internal photoemission and photoconductivity measurements. Two band gap values associated with Ga2O3 (4.8 eV) and Gd2O3 (5.8 eV) subnetworks were revealed. They yield potential barriers between the GaAs valence band and the bottom of the Ga2O3 and Gd2O3 derived conduction bands of 2.2 and 2.9 eV, respectively. The corresponding conduction band offsets at the GaAs/oxide interface, 0.8 and 1.5 eV, indicate the possibility of significant reduction of electron injection in Gd-rich oxides. (C) 2004 American Institute of Physics.