cis-Tetraamminedichlororuthenium(III) (cis-[Ru(NH3)(4)Cl-2](+)) complex was found to work as a very active water oxidation catalyst to evolve dioxygen (O-2) in an aqueous solution as well as in a Nafion membrane. The initial O-2 evolution rate (V-o2, (mol s(-1))) was first-order with respect to the complex concentration in both the solution and membrane. The catalyst undergoes a bimolecular decomposition to deactivate at high complex concentrations. The first-order rate constant (k(o2) (s(-1))) for O-2 evolution and the second-order rate constant (k(deact) (dm(3) mol(-1) s(-1))) for bimolecular decomposition were obtained from the kinetic analysis based on the competition between both processes. The k(o2) (1.4 x 10(-2) s(-1)) in the Nafion membrane is much higher than that (2.3 x 10(-3) s(-1)) in the solution. The k(deact) (5.1 x 10(-1) dm(3) mol(-1) s(-1)) in the membrane was slightly higher than that (2.0 x 10(-1) dm(3) mol(-1) s(-1)) in the solution, The catalytic activity in the Nafion membrane was analyzed in terms of the intrinsic activity (k(o2) (s(-1))) and critical decomposition distance (r(d) (nm)) on the basis of the intermolecular distance distribution to obtain k(o2) = 1.5 x 10(-2) s(-1) and r(d) = 0.88 nm. The k(o2) was compared with the earlier results on [Ru(NH3)(6)](3+) and [Ru(NH3)(5)Cl](2+); the k(o2) of the complexes was in the order cis-[Ru(NH3)(4)Cl-2](+) > [Ru(NH3)(5)Cl](2+) > [Ru(NH3)(6)](3+). The structural effect on the catalytic activity was discussed.