Li-ion insertion and extraction were investigated by electrochemical impedance spectroscopy (EIS) on a single particle mesocarbon microbead (MCMB) (heat-treated at 2800 degreesC) of 30 mum diameter in I 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-in finite 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 deereased/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) cm(2) s(-1), varying as a function of electrode potential with minima at the potentials corresponding to the voltammetric peaks.