The oxidation behavior of a sputtered carbon electrode has been investigated in 0.5 M H2SO4 solution with the help of an electrochemical quartz crystal microbalance (EQCM). The mass change response of the carbon coated Au-QCM during potential cycling showed an increase of mass due to adsorption of water or bisulfate ion just after observation of an anodic peak for the oxidation of hydroquinone. A mass loss due probably to the evolution of CO2 gas was observed at potential above 1.3 V vs. SHE during anodic scan. During cathodic scan, a loss of mass due to release of water or bisulfate ion was accompanied by the reduction of quinone to hydroquinone. By comparing the result of carbon coated QCM with that of highly ordered pyrolytic graphite (HOPG), the mechanism for the oxidation of carbon in acidic medium is discussed.