V SA , but also to the behavior of recently reported anatase TiO2 containing single Cu(II/I) ions occupying the Ti vacancy sites which exhibited fast re-oxidation of reduced Cu(I) sites by oxygen. This indicates that our a-TiO2-Cu SA with surface-grafted Cu(II) SACs is distinct from the material reported by Lee et al. in which the Cu(II) ions act as substitutional dopants, and suggests that the reactivity of Cu(II) SACs towards oxygen is strongly dependent on the local bonding environment of Cu(II) in/at anatase TiO2. Fourthly, these results also raise a question regarding the reasons for the significant enhancement of photocatalytic degradation rates at a-TiO2-Cu SA (see Figure ). As the kinetics of the re-oxidation of Cu(I) to Cu(II) in a-TiO2-Cu SA is rather sluggish, the catalysis of dioxygen reduction (the primary reaction of the reductive pathway), as in the case of a-TiO2(400)-V SA , is unlikely to be the reason. On one hand, our EPR analysis demonstrates that the Cu(II) SAC sites can effectively trap photogenerated electrons, and thus improve the charge separation yield. On the other hand, an efficient photocatalytic process requires that the Cu(I) sites generated after the reduction by photogenerated electrons are continuously re-oxidized during the photocatalyst operation on a timescale kinetically similar to the electron trapping at Cu(II)/Cu(I) sites. In this context, it is important to realize that O2 is not the only oxidizing agent that is available for re-oxidation of Cu(I) in a conventional photocatalytic degradation process. In particular, it is well established that hydrogen peroxide is formed during photocatalysis at TiO2, especially when organic compounds act as hole scavengers as is the case in pollutant degradation reactions studied here. Under such conditions, significant amounts of H2O2 can be formed via various reactions on the reductive pathway initiated by the reduction of dioxygen to superoxide anion radical 2 O -by photogenerated electrons ( CB e -) in the conduction band of TiO2: