High-field and frequency electron paramagnetic resonance (HFEPR) of solid (8,12-diethyl-2,3,7, 13,17,18-hexamethylcorrolato)manganese(III), 1, shows that in the solid state it is well described as an S = 2 (high-spin) Mn(III) complex of a trianionic ligand, [(MnC3-)-C-III], just as Mn(III) porphyrins are described as [(MnP2-)-P-III](+). Comparison among the structural data and spin Hamiltonian parameters reported for 1, Mn(III) porphyrins, and a different Mn(III) corrole, [(tpfc)Mn(OPPh3)], previously studied by HFEPR (Bendix, J.; Gray, H. B.; Golubkov. G.; Gross, Z. J. Chem. Soc., Chem. Commun. 2000, 1957-1958), shows that despite the molecular asymmetry of the corrole macrocycle, the electronic structure of the Mn(HI) ion is roughly axial. However, in corroles, the S = 1 (intermediate-spin) state is much lower in energy than in porphyrins, regardless of axial ligand. HFEPR of 1 measured at 4.2 K in pyridine solution shows that the S = 2 [(MnC3-)-C-III] system is maintained. with slight changes in electronic parameters that are likely the consequence of axial pyridine ligand coordination. The present result is the first example of the detection by HFEPR of a Mn(III) complex in solution. Over a period of hours in pyridine solution at ambient temperature, however, the S = 2 Mn(III) spectrum gradually disappears leaving a signal with g = 2 and Mn-55. hyperfine splitting. Analysis of this signal, also observable by conventional EPR. leads to its assignment to a manganese species that could arise from decomposition of the original complex. The low-temperature S = 2 [(MnC3-)-C-III] state is in contrast to that at room temperature. which is described as a S = 1 system deriving from antiferromagnetic coupling between an S = (3)/(2) Mn(II) ion and a corrole-centered radical cation: [(MnC.2-)-C-II] (Licoccia, S.; Morgante. E.; Paolesse. R., Polizio, F.; Senge, M. O., Tondello. E.; Boschi, T, Inorg. Chem. 1997, 36, 1564-1570). This temperature-dependent valence state isomerization has been observed for other metallotetrapyrroles.