The surface layer on a petroleum coke in tetraglyme solutions of LiAsF6 or LiCF3SO3 was analysed with X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy, Gas Chromatography and fluoride selective electrode. Results indicate that solvent reduction is a minor process and salt reduction a major one. Results are consistent with the reduction mechanism of LiAsF6 proposed in the literature. But with LiCF3SO3, we suggest that reduction proceeds via the cleavage of a CF bond instead of a CS bond.