We describe the use of null-ellipsometry with lock-in detection to monitor grain core oxygen diffusion in thin films of the solid-state ionic conductor Ce0.8Gd0.2O1.9. Application of an electric field perpendicular to the film surface probe alternating voltage (U-A) in addition to perturbation bias voltage (U-B)-produces ellipsometer optical response at the probe frequency. The signal amplitude and time dependence can be interpreted in terms of changes in the local material polarizability. Since the ionic contribution to the material polarizability is much larger than that of electrons or protons, the diffusion of ions can be distinguished. Because grain cores occupy the majority of the film volume, ion diffusion in the grain cores dominates the optical response. This effect was studied as a function of temperature (75-160 degrees C), amplitude and frequency of the electric field. The activation energy for oxygen diffusion in the grain cores was found to be 1.1 +/- 0.1 eV and 1.5 +/- 0.1 eV for films with in-plane compressive strain of 034 +/- 0.06% and in-plane tensile strain 0.16 +/- 0.03%, respectively. (C) 2015 Elsevier B.V. All rights reserved.