The activity of tris(ethylenediamine)ruthenium (III) complex, [Ru(en)(3)](3+), as a water oxidation catalyst was studied in a homogeneous aqueous solution and a heterogeneous Nafion (Nf) membrane. In the aqueous solution, the apparent catalytic activity (k(app) (s(-1))) decreased monotonously with the concentration due to a bimolecular decomposition of the complex. The bimolecular decomposition of the complex was remarkably suppressed by incorporating it into a Nf membrane. An optimum complex concentration for k(app) in the Nf membrane was exhibited, which was explained both by a cooperative catalysis and a bimolecular decomposition of the complex. The k(app) in the Nf membrane was analyzed in terms of an intrinsic catalytic activity (k(O2) (s(-1))) of the complex, a cooperative catalysis distance (r(co) (nm)) and a critical decomposition distance (r(d) (nm)) between them based on intermolecular distance distribution to obtain the k(O2) = 8.5 x 10(-5) s(-1) r(co) =1.44 nm and r(d) =1.07 nm. The results in the [Ru(en)(3)](3+) system were compared with those obtained in the [RU(NH3)(6)](3+) system.