The reduction of CO, by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates. The resonance Raman data of intermediate I, which could only be stabilized at -95 degrees C, indicates that it is a Fe(II)-CO22- adduct and is followed by an another intermediate II at -80 degrees C where the bound CO, in intermediate I is protonated to form a Fe(II)COOH species. While the initial protonation can be achieved using weak proton sources like MeOH and PhOH, the facile heterolytic cleavage of the C-OH bond in intermediate II requires strong acids.