The [Ru(bpy)(3)](2+)/persulfate photosystem is the most common dye sacrificial reagent pair used to study the catalyzed water oxidation half-reaction. Recently, we developed a bubbling reactor along with its modelling, and we used it with the aforementioned photosystem to measure the actual rate of reaction (R-O2) over time. In the present work, the same method is employed to investigate the kinetics of the reaction occurring through several steps, i.e. not only water oxidation, but also parasitic reactions due to chemical instability of the intermediate [Ru(bpy)(3)](3+) species, which degrade over time finally decreasing the reaction rate. O-2 evolution as catalyzed by Mn2O3 is examined at three irradiance conditions, and for three different catalyst contents. Qualitatively, whereas the increase of catalyst amount yields the expected increase of O-2 production and evolution rate, the increase in irradiance enhances the degradation processes, thus giving a "paradoxical" effect of decreasing the chemical yields. Chemical kinetics are applied, and predictions are compared to experimental data derived from bubbling-reactor. The development of the kinetic model imposing steady state condition on transient species yields as expression of R-O2 a simple linear combination of two exponentials. It is found that the main activity indicators obtained from the bubbling reactor tests can be related in an expression that is independent of the amount catalytic sites, i.e. the ratio between the O-2 evolution rate and the total amount Of O-2 evolved during the reaction. Results show that the ratio between the kinetic constants of the desired (i.e. O-2 formation) and undesired path (Le. dye degradation) decreases at increasing irradiance, evidencing how the role of parasitic reactions, far from being negligible, tends to be overwhelming. (C) 2016 Elsevier B.V. All rights reserved.