Oxidation of water to dioxygen by Ru(bpy)(3)(3+) entrapped within the supercages of zeolite Y is the object of this study. The encapsulation and isolation of individual Ru(bpy)(3)(3+) molecules precludes multi-metal centered degradation reactions, typically observed in solution. With the help of diffuse reflectance, resonance Raman, and electron paramagnetic resonance spectroscopies, the mechanism of the reaction has been investigated. The proposed mechanism involves reaction of Ru(bpy)(3)(3+) with water to form a covalent hydrate Ru3+(bpy)(2)(bpy-H2O) which is deprotonated in the presence of base to form Ru3+(bpy)(2)(bpy-OH-). Intramolecular electron transfer leads to the formation of a hydroxylated bipyridine radical and the metal is reduced to Ru(II). The slow step is the dissociation of this complex to form Ru(bpy)(3)(2+) and hydroxyl radical (OH.), which reacts with unreacted Ru(bpy)(3)(3+). Hydrogen peroxide is proposed to be formed in a two-electron transfer step by reaction of hydroxide ion (OH-) with Ru3+-(bpy)(2)(bpy-OH.). The formation of O-2 by reaction of H2O2 with unreacted Ru(III) is proposed to follow similar one-electron steps as in solution. Dioxygen formation occurs almost quantitatively when the Ru(bpy)(3)(3+)-zeolite Y is in contact with basic solution at pH 12, whereas no formation of dioxygen is observed upon exposure to pH 4 solution. The isolation of reactive molecules in zeolite cages allows for a convenient way to study their chemistry and leads to observation of chemical pathways that do not occur in solution.