The electrochemical reaction of lithium and nanocrystalline Co3O4 thin films fabricated by reactive pulsed laser deposition was investigated using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and galvanostatic measurements. Our new finding is that during the first discharge process of nanocrystalline Co3O4 film-based lithium cell Co3O4 thin film reacts with lithium and forms nano-sized metallic Co and Li2O, but after charging to 3.0 V CoO instead of Co3O4 is generated and the subsequent discharge/charge processes are reversible. A reaction mechanism of nanocrystalline Co3O4 thin film with lithium is proposed. (C) 2004 Elsevier B.V. All rights reserved.