In the current chapter, the use of a bubbling reactor for investigating the activity of different manganese oxides (i.e. Mn2O3, Mn3O4 and MnO2) is described. The most important aspects of water oxidation reaction considered are the role of the catalyst specific surface area (SSA), and the effect of the irradiance on the system. The former was investigated by preparing and testing samples with the same crystal structure, but with different SSA values. Notably, water oxidation catalyst activity does not strictly increase with SSA, but rather depends on the preparation route, which affects the nature of the surface. The effect of the irradiance was studied by using three irradiances conditions (i.e. 0.3, 0.5 and 1.0 sun) and three different catalyst contents (i.e. 10, 20 and 40 mg), evidencing how the increase in irradiance enhances the degradation processes rather than the O2 evolution. On this basis, a kinetic model imposing steady state conditions on transient species was developed, yielding a simple linear combination of two exponentials as expression of. The results show that the ratio between the kinetic constants of the desired (i.e. O2 formation) and undesired path (i.e. dye degradation) decreases at increasing irradiance, evidencing how the role of parasitic reactions, far from being negligible, tends to be overwhelming.