Activity of hexaammineruthenium(III) complex ([RU(NH3)(6)](3+)) as a Water oxidation catalyst was investigated in a homogeneous aqueous solution (AS) as well as in a heterogeneous Nafion membrane (HM). In the AS, the apparent catalytic activity (k(app)/s(-1)) decreased monotonically with the concentration, which was ascribed to a bimolecular decomposition of the complex at high concentrations. The bimolecular decomposition was suppressed remarkably by incorporating the complex into the membrane. It was shown that 4-electron water oxidation is cooperatively catalyzed by two molecules of the complex in the membrane. The k(app) in the HM was much higher than that in the AS at the whole concentration range, which was ascribed both to a favorable interaction for the cooperative catalysis and to the suppression of the bimolecular decomposition by immobilization of the complex. The k(app) was analyzed in terms of an intrinsic activity (k(O2)/s(-1)) of the complex, cooperative catalysis distance (r(co)/nm) and critical decomposition distance (r(d)/nm) between them based on the intermolecular distance distribution to give k(O2) = 3.5 x 10(-5) s(-1), r(co) = 1.21 nm, and r(d) = 0.82 nm. These values were discussed and compared with those obtained in the pentaamminechlororuthenium(III) complex ([Ru(NH3)(5)Cl](2+)) system.