In our preliminary communication (Ogo, S.; Wada, S.; Watanabe, Y.; Iwase, M.; Wada, A.; Harata, M.; Jitsukawa, K.; Masuda, H.; Einaga, H. Angew. Chem., Int. Ed. 1998, 37, 2102-2104), we reported the first example of X-ray analysis of a mononuclear six-coordinate (hydroxo)iron(III) non-heme complex, [Fe-III(tnpa)(OH)(RCO2)]ClO4 [tnpa = tris(6-neopentylamino-2-pyridylmethyl)amine; for 1, R = C6H5], which has a characteristic cis (hydroxo)Fe-III-(carboxylato) configuration that models the cis (hydroxo)-Fe-III-(carboxylato) moiety of the proposed (hydroxo)iron(III) species of lipoxygenases. In this full account, we report structural and spectroscopic characterization of the cis (hydroxo)-Fe-III-(carboxylato) configuration by extending the model complexes from 1 to [Fe-III(tnpa)(OH)(RCO2)]ClO4 (2, R = CH3; 3, R = H) whose cis (hydroxo)-Fe-III-(carboxylato) moieties are isotopically labeled by (OH-)-O-18, (OD-)-O-16, (OD-)-O-18, (CH3CO2-)-C-12-C-12-O-18, (CH3CO2-)-C-12-C-13-O-16, (CH3CO2-)-C-13-C-12-O-16, (CH3CO2-)-C-13-C-13-O-16, and (HCO2-)-C-13-O-16. Complexes 1-3 are characterized by X-ray analysis, IR, EPR, and UV-vis spectroscopy, and electrospray ionization mass spectrometry (ESI-MS).