To summarize, while both Cu(II) and vanadate act as effective SACs that enhance the photocatalytic degradation rate, the mechanism of enhancement is both similar and different in each case. The similarity consists mainly in the fact that both Cu(II) and vanadate SACs are easily reducible, and thus capable of effective trapping of electrons photogenerated in TiO2 and enhancing the initial charge separation. The difference is in the reactivity of the reduced catalyst towards dioxygen. The vanadate is easily reoxidized by O2 and acts thus as a SAC for the direct reduction of O2 to superoxide anion radical, i.e. for one of the primary reaction steps in photocatalytic degradation mechanism. In contrast, the reduced Cu(I) does not react with dioxygen but with H2O2, a typical intermediate of photocatalytic degradation mechanism.