Abstract In situ Fourier transform infrared spectroscopy (FTIR) spectra measurements were obtained for a Cu electrode/solution of lithium perchlorate in propylene carbonate (PC). The dependence of potential on the concentration of PC in the vicinity of the electrode was investigated. The bands due to free PC and PC solvated to lithium ions in the solution were distinguished by the single reflection attenuated total reflection (ATR) spectra. In the FTIR spectra, the reversible change in the concentration of free PC and solvated PC in the diffuse double layer was observed to be accompanied by a change in potential. As the potential decreased, the free PC concentration increased, while the concentration of the PC solvated to lithium ions decreased. Thus, it can be concluded that the equilibrium shifts from Li +(PC) 4 to Li +(PC) 3 + PC as the potential decreases. Furthermore, Li +(PC) 3 orientates itself so that it is parallel to the electrode surface.