We investigated the open-circuit potentials ( OCPs) and structure of CoV3O8 in the first discharge and charge processes. The OCPs in the discharge and charge processes traced different routes. In the discharge process, the OCPs declined slowly at similar to 2.4 V and reduced more slowly at similar to 2.2 V. From the change in the cell parameters, lithium was found to be intercalated in the tunnel space of CoV3O8 at low lithium contents. Further lithium insertion induced an amorphous material and a crystalline one. The generation reaction of these substances containing lithium ions resulted in an irreversible capacity in the first cycle. Considering that the observed OCPs were distributed over a wide range of potentials in the charge process, it was probable that lithium was extracted dominantly from the newly generated amorphous substance. We concluded that the amorphous substance exhibited a reversible discharge-charge cycle performance after the second cycle. (c) 2007 The Electrochemical Society.