The water insoluble complex Rb-4[Ru-II(bpy)(3)](5)[{(Ru4O4)-O-III(OH)(2)(H2O)(4)}(gamma-SiW10O36)(2)], ([Ru(II)bpy](5)[(Ru4POM)-P-III]) was synthesized from Rb8K2[{(Ru4O4)-O-IV(OH)(2)(H2O)(4)}(gamma-SiW10O36)(2).] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [Ru(II)bpy](5)[R111114p0M] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)(3)](3+)([Ru(III)bpy]) acting as the electron transfer mediator and [(Ru4POM)-P-v] as the species releasing O-2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s(-1) was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiCIO4 as an additional electrolyte indudes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [Ru(III)bpy](5)[(Ru4POM)-P-III] -> [Ru(III)bpy](5)[(Ru4POM)-P-v] + 13 e(-) and [Ru(III)bpy](5)[(Ru4POM)-P-v] + 2H(2)O -> [Ru(III)bpy](5)[(Ru4POM)-P-IV] + 02 + 4H(+) The voltammetry of related water insoluble [Ru(II)bpy](2)[S2M18O62] (M = W and Mo) and [Fe(II)Phen](x)[(Ru4POM)-P-III] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [Ru(III)bpy] is the electron transfer mediator and [(Ru4POM)-P-v] is the species responsible for oxygen evolution.