The intercalation of Li into layer-structure materials is closely compared with the features of underpotential deposition (UPD). Based on the initiation potential adsorption behaviour, cyclic voltammetric and impedance response of Li X C6, it is proposed that Li intercalation can be considered to be a special type of UPD with the substrate being a layered structure. The thermodynamic behaviour of Li intercalation in graphite can be interpreted in terms of an adsorption isotherm and an inverse derivative curve. The thermodynamic equation of the adsorption isotherm, including the long-range interaction energy between Li atoms, has been used and a close match has been found with the result in the final two phase transitions of cell discharge.