The cathodic reduction of oxygen and the degradation of formaldehyde with hydrogen peroxide electrogenerated in situ was studied at graphite and modified graphite as cathodes. The peak current of the cathodic reduction of oxygen increased significantly when anodically treated graphite and graphite modified with 2,3,7,8,12 13,17 18-octaethyl-21H,23H-porphyrine cobalt (Co(II)OTP) were used as cathodes, respectively. The current density for the oxidative degradation of formaldehyde was significantly increased when graphite was replaced by anodized graphite as the cathode. Using anodized graphite as cathode, the degradation fractions of the formaldehyde were 99.6 and 98.3% in divided and undivided cells, respectively.