It has been shown that the spinel-type cobalt oxide, Co3O4 is electrodeposited laterally along the glass substrate oil a comb-shaped microelectrode from an alkaline solution with Co2+ and glycine and the a.c. impedance of the cobalt oxide-covered microelectrode is considerably affected by the relative humidity. The resistance (R-film) and capacitance (C-film) of the surface film of cobalt oxide were decreased and increased with increasing the relative humidity, respectively. This was attributed to the adsorption of water-vapor molecules onto the Co3O4 film followed by the oxidation of this film. Adsorption of water molecules on Co3O4 gives the hydroxyl group on a surface cobalt ion and the proton forming a second hydroxyl with an adjacent surface oxygen ion. The change in a.c. impedance was therefore concluded to be related to the generation of mobile protons which was brought about by the decomposition of hydroxyl groups induced by the oxidation of divalent cobalt ion to trivalent. (C) 2003 Elsevier Science B.V. All rights reserved.