Sn–Co alloy thin films with high IR reflectivity were prepared by electroplating on a copper substrate and served as anodes of lithium ion battery. The interfacial properties of the Sn–Co alloy anode in an electrolyte of 1M LiPF6/EC+DMC (1:1, vol.%) during discharge/charge (or lithiation/delithiation) processes were investigated by using in situ microscope Fourier transform infrared reflection spectroscopy (in situ MFTIRS). The results demonstrated that the solvation/desolvation reactions of lithium ions with solvent molecules in discharge/charge processes vary with the concentration of both solvated and free solvent molecules, leading to the shift of CO, C–O and C–H IR bands. The effect of solvation/desolvation, which provides a possibility to probe the lithiation/delithiation processes by in situ MFTIRS, is observed and analyzed clearly. The solid electrolyte interphase (SEI) layer on a cycled Sn–Co alloy anode has also been investigated by ex situ MFTIRS, which determined that the main chemical composition of the SEI layer is ROCO2Li. The current studies are of significance in understanding the interfacial reactions involving in lithium ion battery at molecular level.