In order to obtain a deeper insight into the mechanism of photoactivity enhancement at SACmodified photocatalysts, we carried out a series of in-situ EPR experiments that allow for direct observation of changes in the oxidation state of SACs upon photoreduction and subsequent exposure to dioxygen. We focus on the comparative analysis of a-TiO2(400)-V SA and a-TiO2-Cu SA , as these materials exhibited the highest photocatalytic enhancement and the analysis of their EPR spectra allows for a more straightforward interpretation as compared to a-TiO2-Fe SA (see Supporting Information, Figure and Note 1). First, we turn to the bestperforming sample a-TiO2(400)-V SA . After degassing, the sample a-TiO2(400)-V SA showed only a weak EPR signal from monomeric surface vanadyl VO 2+ species in a distorted octahedral environment (Figure ). As expected, UV irradiation resulted in a strong increase in signal intensity upon the reduction of the EPR-silent V(V) to the EPR-active V(IV). This indicates that the surface vanadate species in a-TiO2(400)-V SA can be very effectively reduced by electrons photogenerated in TiO2. Notably, the EPR signal remained virtually unchanged after the irradiation was turned off as long as the system was kept under oxygen-free conditions.