A number of mono(imidazole)-ligated complexes of perchloro(meso-tetramesitylporphyrinato)iron(III), [Fe(TMP)L]-ClO4, have been prepared, and their spin states have been examined by H-1 NMR, C-13 NMR, and EPR spectroscopy as well as solution magnetic moments. All the complexes examined have shown a quantum mechanical spin admixed state of high and intermediate-spin (S = 5/2 and 3/2) states though the contribution of the S = 3/2 state varies depending on the nature of axial ligands. While the complex with extremely bulky 2-tert-butylimidazole (2-(t)Bulm) has exhibited an essentially pure S = 5/2 state, the complex with electron-deficient 4,5-dichloroimidazole (4,5-Cl(2)lm) adopts an S = 3/2 state with 30% of the S = 5/2 spin admixture. On the basis of the H-1 and C-13 NMR results, we have concluded that the S = 3/2 contribution at ambient temperature increases according to the following order: 2-(t)Bulm < 2-(1-EtPr)Im < 2-Melm less than or equal to 2-EtIm less than or equal to 2-(i)PrIm < 4,5-Cl(2)Im. The effective magnetic moments determined by the Evans method in CH2Cl2 solution are 5.9 and 5.0 muB at 25 degreesC for [Fe(TMP)(2-(t)BuIm)]ClO4 and [Fe(TMP)(2-MeIM)]ClO4, respectively, which further verify the order given above. Comparison of the NMR and EPR data has revealed that the S = 3/2 contribution changes sensitively by the temperature; the S = 3/2 contribution decreases as the temperature is lowered for all the mono(imidazole) complexes examined in this study. The solvent polarity also affects the spin state; polar solvents such as methanol and acetonitrile increase the S = 3/2 contribution while nonpolar solvents such as benzene decrease it. These results are explained in terms of the structurally flexible nature of the mono(imidazole) complexes; structural parameters such as the Fe(III)-N-axial bond length, displacement of the iron from the N4 core, tilting of the Fe(III)-N-axial bond to the heme normal, orientation of the coordinated imidazole ligand, etc., could be altered by the nature of the axial ligands as well as by the solvent polarity and temperature. Some mysteries on the spin states of cytochromes c' isolated from various bacterial sources are possibly explained in terms of the flexible nature of the mono(imidazole)-ligated structure.