Li-ion insertion and extraction were investigated by electrochemical impedance spectroscopy (EIS) on a single particle mesocarbon microbead (MCMB) (heat-treated at 2800°C) of 30 μm diameter in 1 M LiClO4/propylene carbonate+ethylene carbonate solution. The impedance spectra were modeled using an equivalent circuit comprising: (i) Li-ion conduction in the solid electrolyte interphase (SEI) film. (ii) Two charge transfer processes related to the particle/SEI interface, and the SEI/electrolyte interface. (iii) A semi-infinite Warburg-type element, reflecting solid state Li-ion diffusion. (iv) An intercalation capacitance reflecting the accumulation of lithium into the particle. The resistance for Li-ion conduction in the solid electrolyte interphase (SEI) film was found to be independent of the potential. On the other hand, the charge transfer resistance decreased/increased monotonously when the electrode potential shifted to cathodic/anodic direction. This behavior is due to an activation process. The apparent chemical diffusion coefficient of lithium in the MCMB particle was found to be within the range 10−6–10−10 cm2 s−1, varying as a function of electrode potential with minima at the potentials corresponding to the voltammetric peaks.