The solid state diffusion of Lithium into graphite during electrochemical intercalation processes was investigated using potentiostatic intermittent titration (PITT) and impedance spectroscopy (EIS). The diffusion coefficient (D) as a function of the intercalation level (X) and the electrode potential (E) was calculated on the basis of both methods and gave similar results. The D vs X or E plots were found to be nonmonotonous, with three pronounced minima at the same potentials in which the cyclic voltammetry of these systems shows the peaks related to the phase transition between the Lithium-graphite intercalation stages. It was found that the critical diffusion length for these electrodes relates to the graphite particles’ dimensions along their basal planes and not to the electrode thickness. The reason for the peaklike dependence of the D on X and E is discussed in light of the nature of the intercalation processes.