The use of Al2O3 as a gate dielectric has been shown to improve the performance and electrical stability of alpha-InGaZnO4 (IGZO) thin film transistors. X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the valence band (Delta E-v) of Al2O3/IGZO heterostructures deposited by sputtering at <50 degrees C, respectively. A value of Delta E-v = 0.95 +/- 0.17 eV was obtained by using the In 2p3/2, Zn 2p3/2 and 3d5/2 energy levels as references. Given the experimental bandgap of 3.2 eV for the IGZO, this would indicate a conduction band offset of 2.35 eV in this system. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.001111esl] All rights reserved.