The initial redox potential is employed to control the quality of expandable graphite prepared from natural flake graphite by chemical oxidation using KMnO₄ as oxidant and sulfuric acid as intercalation agent. The effects of the initial redox potential on the layer structure and the expansion volume of the expandable graphite are investigated by scanning electron microscopy and X-ray diffraction. Results show that there is an optimum initial redox potential range between 1340 and 1819mV, at which the extent of graphite layer oxidation is moderate and the intercalation reaction is complete. Low-order graphite intercalation compounds are formed and a high expansion volume of the expandable graphite can be obtained in this potential range. The redox potential can be used as a quality control parameter for the industrial preparation of expandable graphite.