We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of (Fe3MnII)-Mn-III/II-OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate (Fe3MnIII)-Mn-III/II-OH. Fe-57 Mossbauer spectroscopy reveals that oxidation state changes (three for (Fe3Mn)-Mn-III/II-OH2 and four for (Fe3Mn)-Mn-III/II-OH clusters) occur exclusively at the Fe centers; the Mn center is formally Mn-II when water is bound and Mn-III when hydroxide is bound. Experimentally determined pK(a) (17.4) of the [(Fe2FeMnII)-Fe-III-Mn-II-OH2] cluster and the reduction potentials of the [Fe3Mn-OH2] and [Fe3Mn-OH] clusters were used to analyze the O-H bond dissociation enthalpies (BDEO-H) for multiple cluster oxidation states. BDEO-H increases from 69 to 78 and 85 kcal/mol for the [(FeFe2MnII)-Fe-III-Mn-II-OH2], [(Fe2FeMnII)-Fe-III-Mn-II-OH2], and [(Fe3MnII)-Mn-III-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn-OHx] system was obtained by constructing a potential-pK(a) diagram; the shift in reduction potentials of the [Fe3Mn-OHx] clusters in the presence of different bases supports the BDEO-H values reported for the [Fe3Mn-OH2] clusters. A lower limit of the pK(a) for the hydroxide ligand of the [Fe3Mn-OH] clusters was estimated for two oxidation states. These data suggest BDEO-H values for the [(Fe2FeMnIII)-Fe-III-Mn-II-OH] and [(Fe3MnIII)-Mn-III-OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn=O] in this and related multinuclear systems.