An experiment designed to measure the thickness of an electrochemically active layer on the surface of a thin-film Co3O4 electrode shows that the entire thickness of the film, 40 nm in all, undergoes chemical changes. This was the case when the film was first exposed to pure water and later subjected to +1 V-SCE in 0.01M KOH solution. Upon removal of the applied potential, the film reverted to a state that was nearly identical to the one it had before polarization. (C) 2014 The Electrochemical Society. All rights reserved.