Resonance Raman (RR) spectra are reported for iron(III) tetramesityl porphyrin (TMP) N-oxide and its O-18 and N-15 derivatives, The RR bands assignable to the Fe-O stretching, O-N stretching, and Fe-O-N bending vibrations were observed at 506, 1122, and 743 and 708 cm(-1), respectively. This confirms that the complex has the Fe-O-N bridged structure. The RR bands of the macrocycle such as the CbetaCbeta and CalphaN stretching modes were split into doublets due to lowering of symmetry. The RR band arising from the C-m-phenyl stretching band exhibited a downshift by 4 cm(-1) upon formation of the N-oxide, suggesting considerable distortion of the macrocycle.