The fundamental feature of the formation and reactions of derivatives of porphyrin iron(III) complex, Fe(P), in an organic solution, O, at the interface between an aqueous solution, W, and O was elucidated with the aid of voltammetry for charge transfer at the interface between two immiscible electrolyte solutions and controlled potential difference electrolysis at the interface. The Fe(P) investigated were chloro(octaethylporphyrinato)iron(III) and chloro(tetraphenylporphyrinato)iron(III), and O adopted was 1,2-dichloroethane, DCE. The formation of Fe(P) derivative such as [Fe(P)](+), [Fe(P)](2)O (mu-oxo dimer), [Fe(P)](2)OH+ or Fe(P)Cl at the W vertical bar O interface was found to depend strongly on pH of W, the potential difference at the interface and the concentration of Cl- in W. The oxidation of Fe(P) derivatives in DCE by oxidants in W at the W vertical bar DCE interface was also investigated, and Fe(P)Cl or [Fe(P)](2)O was found to be oxidized much more easily than [Fe(P)](2)OH+ or [Fe(P)](+). Since [Fe(P)](2)O in O facilitated the transfer of H+ from W to O resulting [Fe(P)](2)OH+ or [Fe(P)](+), the transfer of electron due to the oxidation of Fe(P) derivatives was affected with pH of W, and the transfer of electron and that of H+ facilitated by Fe(P) derivatives at the W vertical bar O interface were controlled by one another. (c) 2005 Elsevier B.V. All rights reserved.