Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnOₓ present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me₃TACN)(OMe)₃]⁺ and [(Me₃TACN)₂Mnⁱⁱⁱ₂(μ-O)(μ-CH₃COO)₂]²⁺ (Me₃TACN = N,N',N ''-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnOx nanoparticles (10-20 nm and 6-10 nm, respectively) were generated in the Nafion film. In contrast, the use of [Mn(OH₂)₆]²⁺ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30-100 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., similar to 20 molecules of O₂ per Mn per hour in dark and 40 molecules of O₂ per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnOₓ films made with [Mn(OH₂)₆]²⁺. This corresponds to a TOF > 100 molecules of O₂ per Mn per second for a 10 nm MnOₓ nanoparticle. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnOx and to further evaluate the origin of differences in catalytic activity.