Activity of a pentaamminechlororuthenium(III) ([Ru(NH3)(5)Cl](2+)) complex 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. In competition with water oxidation catalysis, dinitrogen (N-2) evolution by oxidation of its ammine ligands was also observed, whose rate is second order with respect to its concentration, showing that a bimolecular decomposition of the complexes takes place at high concentrations, The bimolecular decomposition was suppressed remarkably by incorporating the complex into a HM. In the HM, k(app) increased with the catalyst concentration at low concentrations below 1.5 x 10(-2) M, indicating that four-electron water oxidation is cooperatively catalyzed by two complex molecules. The k(app) was strikingly higher for HM than for AS in the whole concentration range, which was ascribed to both a favorable interaction for the cooperative catalysis and the suppression of the bimolecular decomposition by immobilization of the complex. The effect of intermolecular distance distribution of the complex in the membrane on k(app) was analyzed in terms of cooperative catalysis distance (r(co)/nm) and critical decomposition distance (r(d)/nm) to give r(co) = 1.21 nm and r(d) = 0.84 nm. The r(co) value obtained might suggest cooperative catalysis through hydrogen bonds between water and ammine ligands.