Barriers to inversion of the highly deformed porphyrin rings in two ferric porphyrin radical cations, [Fe(OMTPP)Cl]SbCl6 and [Fe(OETPP)Cl]SbCl6 where OMTPP and OETPP are dianions of 2,3,7,8,12,13,17,18-octamethyl- and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, respectively, have been determined by the dynamic NMR method. Inversion barrier of [Fe(OMTPP)Cl]SbCl6 is estimated to be 14.8 kcal mol(-1) (72 degrees C) which is higher than that of the corresponding neutral porphyrin complex [Fe(OMTPP)Cl] by more than 4 kcal mol(-1). Increase in barrier to inversion has also been observed in [Fe(OETPP)Cl]SbCl6, though the precise determination is hampered in this case due to the occurrence of site exchange of the ferric ion across the porphyrin ring; the latter process is verified because the, activation free energies obtained from the line shape analysis of the meta and methyl signals are quite close, 18.1 (113 degrees C) and 17.9 (113 degrees C) kcal mol(-1), respectively. Thus, the inversion barrier in [Fe(OETPP)Cl]SbCl6 is supposed to be much greater than 18 kcal mol(-1). Since the inversion barrier in the corresponding neutral porphyrin complex [Fe(OETPP)Cl] is reported to be 15.8 kcal mol(-1), the barrier has increased by much greater than 2 kcal mol(-1). These results are interpreted in terms of the enhanced nonplanarity of the radical cations as compared with the corresponding neutral complexes. The interaction between porphyrin a(2u) and iron d(x2-y2) orbitals in the radical cations, which is expected to be strengthened in the deformed structure, could be one of the reasons for the conformational change.