Electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer occur during the initial charge/discharge cycles of carbon in electrolytes used in Li-ion batteries. This paper describes our approach to characterize the formation of SEI layers on various carbonaceous materials by in situ ellipsometry. Five types of carbon samples (carbon films on glass, pyrolyzed photoresist on silicon, highly oriented pyrolytic graphite and natural graphite) with specular surfaces were characterized by Raman spectroscopy and in situ ellipsometry/electrochemical studies in ethylene carbonate–dimethyl carbonate containing a lithium salt. Raman spectroscopy showed that the carbons films deposited on glass contain broad overlapping peaks from 900 to 1700cm−1, which is indicative of the highly disordered nature of the carbon films. Changes in the ellipsometric parameters, Δ and ψ, were correlated with the formation of the SEI layer during the initial charge (intercalation) process.