Cobalt aluminum phosphate with AFI zeolitic structure (Co-APO-5) catalyzes light-driven water oxidation (WO) when both the ruthenium complex [Ru(bpy)3]2+, as photosensitizer, and persulfate species S2O82–, as sacrificial electron acceptor, are present. Measurements have been run in a flow reactor, allowing the amount of oxygen evolved to be evaluated and, in particular, the initial rate of WO reaction to be measured. The latter has been studied as a function of chemical composition, and some kinetic features have been established. Competitive reactions occur extensively, causing the oxidation of the Ru complex to the detriment of oxygen production. The initial rate of WO increases with the amount of catalyst until turbidity of the suspension sets in. A reaction order close to −1 with respect to persulfate was found, which indicates the occurrence of surface processes involving Co centers at the outer layer of Co-APO-5 particles (accessed by the bulky [Ru(bpy)3]2+ species, at variance with Co species in the core), for the adsorption onto which competition takes place between the Ru complex and persulfate species. A less pronounced negative order (ca. −0.4) for the sensitizer was also observed, for which an interpretation is proposed.