Diffusion of oxygen ions in thin (40 Å) HfO2 gate dielectric films is measured using transient gate currents. The diffusion coefficient is estimated to be ∼1× 10-14 cm2 /s at room temperature, and is observed to be thermally activated with an activation energy of ∼0.52 eV for nanometer scale HfO2 films. The diffusion results are shown to be consistent with those for positively charged oxygen vacancies in bulk samples of similar oxides, and first principle calculation results for oxygen vacancy diffusion in thin HfO2 films. Thus, we demonstrate a method for measuring oxygen diffusion in thin HfO2 films where diffusion measurements by the conventional techniques have been elusive. © 2011 American Institute of Physics.