powder that is often found to exhibit superior performance in photocatalysis of various reactions. The outstanding performance of a-TiO2(400)-V SA is even better apparent when considering the overall degradation yield after 180 minutes of irradiation and the operational stability after performing several photodegradation cycles with the same initial pollutant concentration (Figure ). In this context, it should be noted that while anatase powder modified with vanadate (a-TiO2(400)-V SA ) consistently outperforms P25 (anatase/rutile mixture) in terms of both initial degradation rate and degradation yield (Figure ), the modification of P25 with vanadate using identical protocols did not result in improved performance, and in the case of modification of pristine rutile TiO2 even an inhibition effect was observed (Supporting Information, Figure ). The beneficial effect of vanadate modification is thus highly dependent on the starting TiO2 material´s phase, and the anatase phase is clearly preferable. In other words, our results also suggest that, when attempting to prepare a photocatalyst with performance better than P25 using SACs, it is actually better not to take P25 as the starting material! Notably, all studied materials show a decline in photoactivity during four consecutive photodegradation cycles (Figure ). However, the fact that this decline is observed also in the case of pristine and platinized TiO2 or P25 suggests that, apart from the partial loss of the co-catalyst that can be expected to happen, for example, via reductive dissolution in the case of Cu(II) and Fe(III), also other factors play a role in the deterioration of performance. These factors possibly include, for example, the accumulation of reaction intermediates that effectively poison the degradation reaction at the photocatalyst surface. Indeed, the relative decrease of activity during the cycling of the best-performing a-TiO2(400)-V SA photocatalyst and for P25 is very similar, which indicates that the loss of the co-catalyst is not a major problem. When aiming at practical application, the operational stability issues would have to be addressed by engineering efforts at process intensification using optimized operational parameters and photoreactor design. Since the specific performance of TiO2-based photocatalysts is known to be highly substratedependent, we investigated also two further photocatalytic reactions in order to get a more solid assessment of the advantages and limits of our SAC-modified photocatalysts. Firstly, we investigated the photodegradation of diethyl phthalate (DEP), a toxic pollutant that does not adsorb well onto TiO2 and its degradation kinetics is typically significantly slower than that of 2,4-D. Again, the photocatalytic degradation rate was enhanced at SAC-modified materials a-TiO2-Cu SA , a-TiO2-Fe SA and a-TiO2(400)-V SA by the factors of ~3.9, ~2.9, ~2.8, respectively, as compared to corresponding unmodified TiO2 samples (Figure ). Secondly, the OH radical formation rate determined by the oxidation of terephthalic acid to hydroxyterephthalic acid was also significantly enhanced at all SAC-modified samples, showing the highest rate in the case of a-TiO2(400)-V SA (Figure ). These results show that the beneficial effect of SAC modification on the photocatalytic degradation performance of anatase TiO2 is not confined to specific oxidizable test substrates, but is rather general. This finding is also in line with our assumption that the presence of reducible SACs affects the overall photocatalytic reactivity predominantly by inducing changes in the reductive mechanistic pathway induced by the photogenerated electrons, i.e., by the trapping of photogenerated electrons at SAC-induced surface reactive sites, enhancing the reaction with dioxygen, and/or catalyzing the subsequent redox transformations of reactive oxygen species (e.g., superoxide anion radical 2 O -and H2O2) formed during the photocatalytic process.