Graphite intercalation compounds are formed by inserting guest molecules or ions between sp 2 -bonded carbon layers. These compounds are interesting as synthetic metals and as precursors to graphene. For many decades it has been thought that graphite intercalation must involve host–guest charge transfer, resulting in partial oxidation, reduction or covalent modification of the graphene sheets. Here, we revisit this concept and show that graphite can be reversibly intercalated by non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids). The products are mixtures of graphite and first-stage intercalation compounds. X-ray photoelectron and vibrational spectra indicate that the graphene layers are not oxidized or reduced in the intercalation process. These observations are supported by density functional theory calculations, which indicate a dipolar interaction between the guest molecules and the polarizable graphene sheets. The intercalated graphites readily exfoliate in dimethylformamide to give suspensions of crystalline single-and few-layer graphene sheets. T he intercalation of layered inorganic solids is an old topic of renewed interest as chemists seek to develop synthetic routes to single-layer graphene and other nanosheet materials. Graphite intercalation was first discovered in 1840 by Schafhäutl, who observed the formation of 'blue graphite' upon reaction with sulfuric acid and oxidizing agents 1 . In 1855, Brodie found that a mixture of sulfuric acid and potassium chlorate or nitric acid pro-duced a lamellar oxide of graphite 2 . Since that time, numerous studies have shown that graphite can be intercalated by oxidizing or reducing agents 3,4 and only one earlier report suggests the possibility of intercalation without adding an oxidizer 5