In the work the intercalation of H2SO4 (from. 98% to 76%) into graphite (oxidizer -(NH4)(2)S2O8) was investigated by potentiometry. It was established that the redox potential of oxidizer solution depends on the concentration of H2SO4 The criterion for the formation of graphite hydrogenosulfate(1) (GHS) was corroborated by the experiment. It was shown that stage of GHS is determined by the redox potential of the oxidizer solution. The system C-K2Cr2O7-94% H2SO4 was investigated by calorimeter and potentiometer methods in situ. The "overoxidation" of stage 1 GHS was observed during the calorimetric investigation (the second exo-effect) but not seen on the potentiometry curve. It was proposed that more high oxidation of graphite and the formation of ion-covalent bonds take place during "overoxidation".