Lithium intercalation and deintercalation processes occur through solid electrolyte interfaces (SEIs) on the zigzag and armchair faces of graphite. Such SEIs are formed by reaction of surface groups on the graphite with lithium upon charging. The nature of this interface to a large extent determines the reversible and irreversible capacities of the graphite. We have investigated the influence of mild oxidation of samples of natural graphite and carbon nanotubes on the surface characteristics of the films formed on these materials upon lithium intercalation. The surface groups formed upon oxidation have been characterized using Fourier transform infrared (FT-IR) spectroscopy. The reversible and irreversible capacities of the thermally oxidized samples are discussed in terms of the surface film composition as well as enhanced surface area that contributes to increased number of sites for lithium intercalation.