Adlayers of cobalt(II) porphine (COP) and [2,3,7,8,12,13,17,18-octaethyl-21H,23-H-porphine]cobalt(II) (CoOEP) were formed on Au(111) by immersing the substrate in a benzene solution containing either CoP or CoOEP molecules and investigated in 0.1 M HClO4 by using in situ scanning tunneling microscopy. Highly ordered arrays of COP were observed on the Au(111)-(1x1) surface under controlled potential conditions. The adlayer of CoOEP molecules was also highly ordered, whereas the Au(111) substrate was found to become reconstructed upon adsorption of CoOEP. Both COP and CoOEP molecules were closely packed on Au(111). On the adlayers of COP and CoOEP, the electrochemical reduction Of O-2 was examined in 0.1 M HClO4 saturated with O-2. The adlayers of both COP and CoOEP on Au(111) enhanced the reduction Of O-2.