The catalytic activity of a trinuclear Ru complex; ([(NH3)(5)Ru-O-Ru(NH3)(4)-O-Ru(NH3)(5)](6+)) (Ru-red) for water oxidation was investigated both in a homogeneous aqueous solution system (AS) and a heterogeneous montmorillonite adsorbed system (clay). It was shown that water oxidation by the catalyst in the clay to evolve O-2 is a competitive reaction with bimolecular decomposition of the catalyst. The bimolecular decomposition was remarkably suppressed in the clay as compared with the aqueous solution, and the catalytic activity was maintained under higher concentration conditions. This was ascribed to the immobilization and isolation of the complex in the clay. The catalytic activity was analyzed by a model equation based on intermolecular distance distribution of the complex. The catalytic activity (k(O2)) and the critical decomposition distance (r(d)) were obtained as 1.7 x 10(-2) s(-1) and 2.39 nm, respectively.