The effect of imidazole and pentachlorophenol (PCP) on coordination and redox properties of a Fe-porphyrin catalyst were studied by EPR and UV-Vis spectroscopy. In the EPR spectra three different Fe-porphyrin species were resolved, corresponding to (a) FeR4P-Cl (S = 5/2, g(x) = 6.13, g(y) = 5.77, g(z) = 1.99), (b) FeR4P-[Imidazole](1) (S = 5/2, g(x) = 6.80, g(y) = 5.20, g(z) = 1.99), and (c) FeR4P-[Imidazole](2) (S = 1/2, g(1) = 2.85, g(2) = 2.30, g(3) = 1.56). The stability constants logK for FeR4P-[Imidazole](1) and FeR4P-[Imidazole](2) have been estimated. PCP affects the redox potential of the Fe-porphyrin species. The system FeR4P/Imid/NaIO4/PCP behaves as an integral entity with redox potential appropriate for efficient redox cycling of the iron. (C) 2010 Elsevier B. V. All rights reserved.