Effects of electrolyte concentrations and the level of preintercalation (x values in LixC6) on the lithium ion intercalation into graphite lattices have been examined in propylene carbonate-ethylene carbonate mixed solutions with LiClO4 as an electrolyte by ac impedance measurement techniques. Exchange current densities were determined for reductive intercalation of lithium by ac impedance measurements to range between 1.4 and 2.4 mA/cm(2) depending on the amount of intercalated lithium ions with a transfer coefficient (alpha) of 0.65. Diffusion coefficients during the deintercalation process have also been determined at various preintercalation levels. The dependence of diffusion coefficients and exchange currents on the x values in LixC6 (x less than or equal to 1) is discussed.