Nonstoichiometric, anhydrous NiOx thin films were produced by sputtering. They can be bleached by means of hydrogen insertion equally fast and efficiently either in an electrochemical cell or in a gas chamber, as demonstrated in this work by using a Pd covered thin film electrode. The electrode mechanical stress and transmittance changes during H insertion and extraction, measured in situ, showed an increase in the compressive stress in the cathodic (bleaching) cycle and a relaxation of the system in the anodic (coloring) process. These experiments demonstrate that the aqueous solvent plays no major role on thean coloring-bleaching process of NiOx thin films. © 1991, The Electrochemical Society, Inc. All rights reserved.