Studying electron transfer reactions of porphyrin molecules is important for a wide range of applications including biology, molecular devices, artificial photosynthesis, information storage, and fuel cells. It is known that porphyrins adsorbed in a self-assembled monolayer at an electrochemical interface may lose their electrochemical activity. However, the mechanism of the suppressed electrochemical activity is not clear. In this article, the electrochemical behavior of the two-dimensional network structures of 5,10,15,20-tetrakis(4-carboxylphenyl)-21H,23H-porphyrin (TCPP) molecules, formed via intermolecular hydrogen bonding on Au(111), was investigated by electrochemical scanning tunneling microscopy (ECSTM). Three types of domains, including a square network with molecules trapped inside, square packing, and hexagonal close-packing structures have been observed under various pH conditions. The difference in STM contrast between oxidized and reduced TCPP allows the slow electrochemical reduction of adsorbed TCPP to be visualized by STM. For the first time, the pH dependent reduction of porphyrins was imaged by EC-STM, revealing the mechanism of porphyrin slow reduction at electrochemical interfaces. TCPP reduction can be accelerated either by tuning the working electrode potential to a more negative value or by lowering the H+ concentration. A redox reaction model was proposed based on the pH dependent reduction of TCPP to elucidate the fundamental aspects of porphyrin redox reactions.