The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) on Au(111) electrodes was investigated. The adlayer structure was found to depend on the electrode potential. At positive potentials (>0.5V(SCE)), a disordered layer of TPyP is formed on the Au(111) electrode. STM images showed that the disordered molecules are immobile. At negative potentials (-0.2V(SCE)), however, the molecules are highly mobile and can no longer be imaged by STM, though they remain on the surface. At intermediate potentials (-0.2 to +0.2V(SCE)), the TPyP formed a highly ordered adlayer. Once the ordered adlayer is formed, it persists even after the potential is stepped to higher values (0.5-0.8 V-SCE). These results can be explained by the role of potential modulated adsorbate-substrate interaction and surface mobility, This suggests the intriguing prospect of using electrode potential to tune surface interactions and to drive surface processes, e.g., molecular self-assembly, in electrochemical systems.