However, the exposure of the a-TiO2-V SA sample to 100 Torr of O2 at room temperature led practically immediately to a strongly diminished signal with roughly the same intensity and shape as before the irradiation (Figure ). This demonstrates that the photoreduced surfacebound V(IV) species are readily and quickly re-oxidized by dioxygen. The observation that the surface vanadate species can be easily reduced by photogenerated electrons and subsequently readily re-oxidized by oxygen suggests that, under photocatalytic conditions, the vanadate species can act as a very effective single-atom redox catalyst for one-electron dioxygen reduction by photogenerated electrons. In turn, the faster rate of oxygen reduction by photogenerated electrons catalyzed by the vanadate SACs reducec the probability of charge recombination, and leads to enhanced photocatalytic rates. Next, we turn to the analogous in-situ EPR study of the Cu(II) ion-modified sample a-TiO2-Cu SA (Figure ). The intensity of the signal originating from isolated Cu(II) sites decreased upon UV irradiation to ~77 % of the initial intensity, indicating the reduction of a portion of single Cu(II) ions to Cu(I) by electrons photogenerated in TiO2. The EPR spectrum after UV irradiation can be simulated by the same components as described above (see Figure ), however with a higher contribution of R-1 and Ti-1 centers, corresponding to the electrons trapped at oxygen vacancies and Ti 3+ centers, respectively. Similarly to the case of a-TiO2(400)-V SA , upon switching off the light no change in the EPR signal was observed. However, in stark contrast to the behavior of a-TiO2(400)-V SA , after exposing the sample to dioxygen the signal intensity was only partially recovered, and reached only ~85 % of the initial intensity. This suggests that, in contrast to a very fast re-oxidation of V(IV) to V(V) in a-TiO2(400)-V SA , the majority of the photoreduced single Cu(I) sites in a-TiO2-Cu SA cannot be readily re-oxidized by dioxygen. In order to further investigate the distinct behavior a-TiO2-Cu SA in more detail, we conducted a series of in-situ EPR experiments whereby the initial photoreduction was carried out in the presence of methanol vapor (50 Torr) (Figure ). As methanol can act as an efficient scavenger of holes photogenerated in TiO2, we assumed that the yield of reduced Cu(I) sites might be higher. Indeed, in the presence of methanol vapor, nearly half of the initial signal of Cu(II) disappeared. After the evacuation of the EPR tube and the addition of O2, the signal remained practically the same (~52 %), which again confirms that the reduced Cu(I) species in a-TiO2-Cu SA do not react readily with dioxygen at room temperature. However, after heating the sample to 150 °C in the presence of O2, the initial signal of Cu(II) was almost completely (~96 %) recovered. Several points are noteworthy in this context. Firstly, it is the fact that the absence or presence of an additional hole scavenger (methanol) has a strong impact on the yield of the photoreduced Cu(I) species. In this context, we note that practically the complete reduction of Cu(II) to Cu(I) was observed during an EPR analysis of the a-TiO2-Cu SA photocatalyst in a liquid water/methanol mixture (Supporting Information, Figure ). The strong EPR signal (g||  2.348, A|| 120 G and g⊥  2.065, A⊥ -not resolved) visible prior to UV irradiation completely vanished after only 15 minutes of UV irradiation. At the same time, the intensity of the EPR signals characterized by g = 2.004 (i.e., electrons trapped at oxygen vacancies, R-1), and g||  1.935 and g⊥  1.970 (i.e., Ti 3+ centers, Ti-1) increased significantly. It should be noted that the signal of Cu(II) disappeared completely although oxygen was not removed from the mixture during this experiment, which again confirms that the re-oxidation of reduced Cu(I) by oxygen is very slow as compared to the reduction. Secondly, the observation of the nearly complete recovery (~96 %) of the Cu(II) signal after increasing the temperature to 150 °C indicates not only that the Cu(II)/Cu(I) single-atom sites are stable under the reduction/re-oxidation cycle, but also suggest that the re-oxidation of Cu(I) to Cu(II) in a-TiO2-Cu SA by dioxygen requires a mild thermal activation step. This, thirdly, is in stark contrast not only to the very fast room-temperature re-oxidation of V(IV) to V(V) by O2 in a-TiO2(400)-