Activity of trans-tetraamminedichlororuthenium(III) complex as water oxidation catalyst was studied both in a homogeneous aqueous solution (AS) and in a heterogeneous Nafion membrane (HM) using Ce(TV) oxidant. This trans-[RuCl2(NH3)(4)](+) was more active than any other mononuclear ammine ruthenium complex studied before. In the AS, the rate of evolved dioxygen is second order with respect to the complex concentration, indicating that the complexes oxidize water molecules bimolecularly in low concentration regions. In high concentration regions, the activity of water oxidation decreased with the increase of the concentration, and at the same time dinitrogen (N-2) evolved by a bimolecular decomposition process. In the HIM, the activity of the complex was remarkably high, and it was maintained for a long time at low concentrations (<0.02 mol dm(-3)). At middle concentrations for the HM (0.02-0.05 mol dm(-3)), the activity of the complex decreased with increasing concentration because of bimolecular decomposition. However, in high concentration regions the complex recovered the activity most probably due to polymerization of the complex. The activity in the HM was analyzed in terms of the cooperative catalysis, the bimolecular decomposition and the polymerization of the complex based on intermolecular distance distribution. <(c)> 2001 Elsevier Science B.V, All rights reserved.