Resonance Raman (RR) spectra are reported for a series of ferric complexes of meso-tetramesitylporphyrin XFe(III)(TMP) (where X = F-, Cl-, m-chlorobenzoate (mCB(-)), CF3O2SO- (trif(-)) and ClO4-) and their oxidized derivatives, OFeIV(TMP(.+))(X). Mode assignments are made by RR studies of the beta-pyrrole-deuterated and O-18-labeled analogues. The results demonstrate that the oxo-iron bond strength is sensitive to the nature of the transaxial ligand (X), the nu(Fe=O) stretching mode appearing near 800 cm(-1) for the F-, Cl-, and mCB(-) complexes and near 835 cm(-1) for the trif(-) and ClO4- complexes. The high-frequency marker modes and the H-1 chemical shifts are consistent with stronger electron donation by the former three anions. An explanation for the apparent absence of a correlation between the nu(Fe=O) stretching frequencies and the previously reported rate constants for substrate epoxidation is also suggested.