The influence of carbonates on TiO2 photocatalysis and the photo-Fenton process using Fe-0 as catalyst is unraveled. The aggregation behavior of each catalyst was monitored under different reaction conditions in order to assess the effect of carbonates on their performance. Carbonates drastically reduced the efficiency of photocatalysis because of aggregation of the TiO2 catalyst, which led to the reduction of adsorbed pollutants. The addition of carbonates to the photo-Fenton process did not produce this strong catalyst aggregation. Therefore, oxidation-reduction reactions on the catalyst surface were not much reduced, and the efficiency of this process was not as strongly affected as that of TiO2 photocatalysis.