We have investigated the possible effect of carboxymethylcellulose (CMC) in the SEI film formation at the surface of a graphite composite electrode of LiCoO2/graphite cells. The electrode/electrolyte interface was analyzed by XPS at different potentials of the first electrochemical cycle, and after simple contact of the electrode with the electrolyte. We could evidence a specific reactivity of CMC towards the electrolyte (LiPF6 in a mixture of carbonate solvents), resulting in the formation of new species that contribute to the surface film composition. This result shows that the chemical reactivity of CMC towards the electrolyte takes part in the formation of the surface film, and contributes to the good properties of CMC as binder.