The adsorption of redox-active 5,10,15,20-Tetrakis(4-trimethylammoniophenyl) porphyrin tetra(p-toluenesulfonate) molecules, abbreviated as [H(2)TTMAPP](4+), on a chloride pre-covered Cu(111) electrode surface was studied in an electrochemical environment by means of combined cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). The [H(2)TTMAPP](4+) molecules adsorb spontaneously on the c(p x root 3) Cl/Cu(111) surface and form a highly ordered 2D monolayer once the supporting electrolyte (10 mMHCl) is replaced by the molecule containing electrolyte (10 mM HCl + 0.1 mMH(2)TTMAPP) at a potential of E = +20 mV vs. RHE (reversible hydrogen electrode). This ordered phase undergoes a phase transition into a disordered one once the[H(2)TTMAPP](4+) molecules pass through the first reduction step at negative potentials. In contrast, at positive potentials, the copper corrosion process, taking place at step-edges with predominant <(2) over bar 11 > directions, is not prevented by the presence of the [H(2)TTMAPP](4+) adlayer.