Mixed-valence states of mu-oxo-centered trinuclear iron carboxylate complexes, [Fe3O(CH2BrCO2)(6)(H2O)(3)] (1), [Fe3O(CH3CO2)(6)(H2O)(3)](.)2H(2)O (2), and [Fe3O(CH2ClCO2)(6)(H2O)(3)]. 3H(2)O (3), were characterized by X-ray crystallography and Fe-57-Mossbauer spectroscopy. Compound 1 has a trapped-valence-type structure, i.e., one Fe(II)-type atom and two Fe(III)-type atoms. The central Fe3O atoms construct nearly an isosceles triangle array with Fe-O bond distances of 1.86(1), 1.83(1), and 2.00(1) Angstrom at 293 K. In the case of compound 2, the X-ray structure shows good equivalence in geometry around the three iron atoms. Results of X-ray crystallography of 1 and 2 are consistent with the results of Mossbauer spectroscopy. The Mossbauer spectra of 1 show a trapped-valence state below 296 K. The Mossbauer spectra of compound 2 are simulated using a three-site relaxation model where an increased rate of intramolecular electron transfer is assumed among the three iron atoms with an increase in temperature. Black crystals of 3 were converted to red powders of [Fe3O(CH2ClCO2)(6)(H2O)(3)] (4) due to loss of all three crystal water molecules when kept in air at ambient temperature. Mossbauer spectra of the dehydrated compound 4 show a temperature-independent trapped-valence state. On the other hand, there are three quadrupole doublets in the Mossbauer spectrum of the hydrated compound 3 measured at 86 K, one of which was ascribed to an Fe-II state and the remaining two to an Fe-III (Fe-IIIA and Fe-IIIB) state. It is found that two of the three doublets (Fe-II and Fe-IIIA) converge without appreciable line broadening with an increase in temperature, while the other doublet (Fe-IIIB) remains unchanged, indicating valence delocalization over the two iron atoms. The X-ray structure of compound 3 also indicates that valence delocalization takes place only between two iron atoms. At 112 K the central Fe3O atoms array with the Fe-O bond distances of 2.003(2) (Fe1), 1.853(2) (Fe2), and 1.849(2) Angstrom (Fe3). A temperature increase to 293 K changes the bond distances to 1.957(3) (Fe1) and to 1.895(3) Angstrom (Fe2), while the bond distance of Fe3-O is almost unchanged (1.846(3) Angstrom). There are intra- and intermolecular hydrogen bonds formed by the crystal water and coordinated water molecules in compound 3. The results suggest that hydrogen-bonding interaction affects the local environment of iron atoms and induces valence delocalization between the two iron atoms.