The catalytic cycle of cytochrome P 450 includes four stable states: a low spin ferric resting state A, a substrate bound high spin ferric state B, a high spin ferrous state C and a low spin ferrous oxy state D. When the system in the state C is exposed to carbon monoxide instead of oxygen a low spin ferrous carboxy state D′ is generated. Several models have been proposed for states B, C, D′ and recently state D [1]. A large variety of spectroscopic techniques have been used to probe the similarities between the model compounds and the actual enzymatic states. Resonance Raman spectroscopy (RR) is a very sensitive technique to investigate specifically the active site of hemoproteins. RR spectra have been obtained for Cyt P 450 from various origins [2–4]: when compared to those of other hemoproteins, the frequencies of the so-called ‘oxidation state’ marker band [5] are unusually low in states C and D′; ‘spin marker bands’ frequencies [5] have been used to monitor the coordination of the iron atom in the ferric states A and B. Recently the FeS stretching mode has been detected at 351 cm −1 in oxidized Cyt P 450 CAM [6]. We report here the results of a RR study of [Fe II(T pivPP)(X −)(L)]18C6Na +* complexes, as models for states C, D′ and D (an investigation of a state A model has already been published [7]). Using the nomenclature propsed in [8], the main RR frequencies are given in Table 1. t001 RR Frequencies (cm −1) of [Fe II(T pivPP)(X −)(L)]Na +18C6 Complexes. Complex Porphyrin vibration FeL vibr. X − L A B C D C 6HF 4S − 1341 a a a 369 Cl − 1343 1355 1494 1545 369 OH − 1344 1355 a a 371 C 6HF 4O − 1343 1354 a 1545 369 C 6HF 4S − CO 1364 a 1567 380 479 C 6HF 4S − O 2 1366 a a 379 a Not observed All the pentacoordinated ferrous species exhibit very similar porphyrinic frequencies. They compare well with the frequencies of the typical high spin ferrous complex Fe(TPP)(2-Me Im) (A = 1345, B = 1361, C = 1500 and D = 1538 [8]). Moreover the A frequency of the carboxy adduct is very close to that of Fe(TPP)(py)(CO) [12], whereas that of the oxy adduct is the same as that of Fe(T pivPP)(1-Me Im)(O 2) [9]. Therefore our RR data do not stress any special π donor properties of the RS − ligand that would induce an extra lowering of the oxidation state marker band frequencies. Soret excitation of the low frequency RR spectrum is readily accessible for the carboxy adduct λ max Soret 448 nm, λ exc 454,5 nm: it reveals a new strong polarized band at 479 cm −1. The intensity of this band decreases with partial photodissociation of the CO ligand. An isotopic substitution experiment, using 13CO, induces a 5 cm −1 lowering of its frequency. This is in good agreement with a calculated shift of −5 cm −1 for the stretching vibration of the FeCO moiety, using the harmonic oscillator approximation. This leads to the assignment of this band to the FeCO stretching vibration. This value is to be compared to those observed for MbCO, HbCO [10] and P 450CO (work in progress).