The adlayer of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine iron(III) chloride (FeOEP) was prepared by immersing Au(111) substrate in a benzene solution containing FeOEP molecules, and it was investigated by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) to examine the effect of the adlayer on the electrocatalytic activity for the reduction of O-2 in 0.1 M HClO4. Voltammetric results indicated that the FeOEP-modified Au(111) electrode enhanced the two-step, four-electron reduction of O-2 to H2O. In situ STM images revealed the highly ordered adlayer of FeOEP on the reconstructed Au(111) surface during the first electrochemical reduction of O-2 to H2O2. When the electrode potential was scanned to the potential region where H2O2 was reduced further to H2O, the STM image became unclear because of the high mobility of FeOEP molecules on the terrace. The result in the present study suggests that the mobility of FeOEP on the Au surface is an important factor affecting the overall four-electron reduction of O-2.