A detailed characterization of the structural modifications and redox processes occurring upon lithium deintercalation from the Li x0Co1−y Mg y O2 materials (x 0=1.0 and 1.10; y=0.0, 0.03, 0.05 and 0.06) was performed in order to determine the effect of Mg doping on the cycling properties. Using electrochemical tests, X-ray diffraction (XRD), 7Li MAS NMR and electrical properties measurements, we show that the Li x Co1−y Mg y O2 system exhibits a solid solution existing in the whole deintercalation range studied (0.30≤×≤1.0). These phases exhibit reversible capacities equivalent to that of LiCoO2 upon cycling with a good structural stability. Moreover, the 7Li MAS NMR study shows that the structural defects (O vacancies and intermediate spin Co3+ ions) which are present in the starting Mg-doped phases govern the electronic properties upon lithium deintercalation. Indeed, regardless of the presence of Mg ions in the structure, a behavior similar to that of the Li x CoO2 (1<x 0≤1.08) system is evidenced with an insulator to metal transition taking place at the microscopic scale.