Measurements have been made of the current excited across Al2O3insulating barriers by proton bombardment. Films of Al2O3sandwiched between thin metal electrodes were bombarded by a monoenergetic beam of protons from a low‐energy accelerator. The Al2O3was grown by anodic oxidation on films of Al which had been deposited on glass substrates. The Al2O3films ranged in thickness from 100 to 600 Å. Ag counterelectrodes deposited on top of the Al2O3completed the junctions. The thickness of the Ag counterelectrode was <500 Å for all of the junctions studied. Current‐voltage characteristics of the junctions were measured while the junctions were in the proton beam. The current‐voltage curves were approximately linear, showing a net electron current from the Ag electrode to the Al electrode at zero bias, and zero electron current when the Al electrode was biased from 2.1 to 4.75 V negative with respect to the Ag electrode. Proton beam currents ranged from 5×10−10to 2×10−11A. Proton energies ranged from 26 to 102 keV. Results are interpreted in terms of a model in which the insulating film is represented by a potential barrier separating a sea of electrons on either side of the barrier. Conduction takes place by passage of excited electrons across the top of the barrier. The effect of the bias is to modify the internal electric field in the oxide and to shift the momentum distribution of the excited electrons. Time‐dependent effects in the current‐voltage curves are observed which are believed to arise from charging of the oxide by the proton beam.