The electrochemical performance of non-graphitized petroleum cokes has been improved by mild oxidation using hydrogen peroxide, a procedure used for the first time in these materials. For this purpose, various carbonisation temperatures and H2O2 treatments were tested. For low sulfur content cokes, the aqueous oxidative treatment significantly increases the capacity values above 372mAh/g during the first cycles. In contrast, cokes with a sulfur content of ca. 5%, did not shown a real improvement. The former results have been interpreted in terms of an effective oxidation of the particles surface, which removes unorganized carbon, where lithium can be irreversibly trapped. Moreover, a stable and less resistive passivating layer grows during the first discharge of lithium, as revealed by impedance spectroscopy. Therefore, chemical procedures, as mild oxidation, open an interesting field of research for the improvement of disordered carbons as anode materials in lithium ion batteries.