The surface coordination chemistry of carbon monoxide with the reduced form ((FePP)-P-II) of iron(III) protoporphyrin IX ((FePP)-P-III) monolayer self-assembled on a Au electrode in 0.1 M HClO4 was studied for the first time by using in situ ATR-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Both mono- and biscarbonyl adducts [simplified as Fe-II(CO)PP and Fe-II(CO)(2)PP, respectively] were detected, depending on the history of potential control. Initially, the Fe-II(CO) PP predominates, and the intermediate transition potential for the conversion of Fe-II(CO)PP to (FePP)-P-III and CO was spectrally determined to be ca. 0.09 V (vs SCE). The ratio of Fe-II(CO)(2)PP and Fe-II(CO) PP increases after a potential excursion to a sufficiently positive value. Fe-II(CO)(2)PP is much more stable against its electro-oxidation to (FePP)-P-III than its counterpart Fe-II(CO)PP with increasing potential. The observed change of coordination properties may be ascribed to an irreversible structural reorganization of the FePP adlayer caused by the potential excursion.