X-ray, IRS, XPS, EDRS and magnetic measurements were used to study Li1+xCoO2 (0 < x greater than or equal to 0.1) samples prepared by conventional ceramic method. It was shown that nonstoichiometric Li1+xCoO2 are characterized by homogeneous crystal structure with statistically distributed vacancies in the cobalt and oxygen layers and the increased Co-O bond covalency. The excess lithium results not in the reduction of Co3+ ions, but in the appearance of a new state of oxygen ions different from cell oxide, with higher value of binding energy (BE), i.e. with smaller electronic density. Acid treatment of Li1+xCoO2 leads to the appearance of delocalized (itinerant) electrons. The electronic state of cobalt ions does not change noticeably whereas the additional oxygen state increases significantly, thus, evidencing that oxygen ions do compensate for the charge upon chemical delithiation. The structure of nonstoichiometric samples appeared to be more stable upon this process. (C) 2003 Elsevier Science B.V. All rights reserved.