An ac impedance spectroscopic study of LiMg_0.05Co_0.95O₂, prepared by sol–gel process, is presented. The results are interpreted on the basis of the equivalent circuit proposed for LiCoO₂ oxide that takes into account the ionic and electronic transport processes. The results demonstrate that the intercalation/deintercalation reaction occurs through two-steps processes involving an adatom mechanism that may be described in terms of partially desolvated lithium ions adsorbed/desorbed from the electrode surface and insertion/loss of an electron from the conduction band of the host followed by diffusion of Li⁺ to/from the lattice intercalation site. Mg doping of LiCoO₂ affects the electrochemical and electronic properties of the pristine oxide by facilitating the insulator-to-conductor transitions that are responsible of the phase transition in the undoped material.