A detailed characterization of the structural modifications and redox processes occurring upon lithium deintercalation from the Lix0Co1-yMgyO2 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), Li-7 MAS NMR and electrical properties measurements, we show that the LixCo1-yMgyO2 system exhibits a solid solution existing in the whole deintercalation range studied (0.30 less than or equal to x less than or equal to 1.0). These phases exhibit reversible capacities equivalent to that of LiCoO2 upon cycling with a good structural stability. Moreover, the Li-7 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 de intercalation. Indeed, regardless of the presence of Mg ions in the structure, a behavior similar to that of the LixCoO2 (1 less than or equal to x(0) less than or equal to 1.08) system is evidenced with an insulator to metal transition taking place at the microscopic scale.