The interfacial phenomena between Cu electrode and solution of lithium perchlorate in ethylene carbonate (EC)–dimethyl carbonate (DMC) have been investigated using in situ reflection absorption Fourier transform infrared (FTIR) spectroscopy and single reflection ATR–FTIR spectroscopy. The ATR spectra confirmed the bands due to free EC and DMC and the molecules solvated to lithium ions in the solution. The bands due to the result of the interaction between ClO4 − and DMC in the mixture solution also appeared in the ATR spectra. In the FTIR spectra, the potential dependence on the concentration of EC and DMC in the vicinity of the Cu electrode was observed. It was understood that the reversible changes in the concentration of free EC and DMC and solvated EC and DMC in the diffuse double layer take place with changing in potential. As the potential decreased, the free EC and DMC concentrations increased, while the concentration of the EC and DMC solvated to lithium ions decreased. Thus, it can be concluded that the equilibrium shifts from Li+(EC)2(DMC)2 to Li+(EC)2(DMC)+DMC or Li+(EC)(DMC)2 +EC as the potential decreases. The bands due to (CH2OCO2Li)2 and CH3OCO2Li were observed for an irreversible reaction.