XPS analyses (core peaks and valence spectra), under highly controlled conditions, have been carried out on stoichiometric LiCoO 2 and lithium-overstoichiometric Li 1+y Co 1-y O 2-y (y ∼ 0.05) materials, with significant changes observed in the oxygen peaks. Indeed, beside the component attributed to the O 2anions of the crystalline network, a second one with variable intensity has been observed on the high binding energy side. With the support of ab initio biperiodical calculations on LiCoO 2 , we propose that this peculiar oxygen signature is partially associated, for LiCoO 2 , to undercoordinated oxygen atoms coming from (0 0 1) oriented surfaces. These surface oxygen anions are significantly less negative than the ones of the lattice. These results, in conjunction with SEM analyses for the lithium overstoichiometric material (as prepared and thermally treated), show that the presence of defects (oxygen vacancies) has to also be considered in the overstoichiometric case. As in battery material, all reactions (the intercalation but also the parasitic ones) occur through the surface; characterization of its crystallographic nature (as well as its electronic properties) is a key point to a better understanding and optimization of Li ion batteries.