Ionic diffusion currents measured in a metal‐insulator‐metal structure yield an activation energy of 1.60 eV at 550°K for alumium ions diffusing into alumium oxide. Aluminum evaporated at 10−10Torr on aluminum oxide, formed by chemical reaction of pure oxygen with the base electrode, produces a structure with a graded alumium‐concentration profile at the lower interface and a relatively abrupt aluminum‐concentration profile at the upper interface. Thermal activation of aluminum ions from the electrodes produces a transient current of diffusing metal ions when an external short circuit prevents charge accumulation on the electrodes. Analysis of the diffusion‐current data of aluminum into aluminum oxide gave values of 1.60 eV for activation energy and 5.8×10−19cm2/sec for the diffusion constant.