Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) represents an attractive approach towards enhancing the photocatalytic performance. However, our knowledge on the mechanism of photocatalysis enhancement at SAC-modified photocatalysts is still rather limited, which makes the rational design of high-performance photocatalysts based on SACs challenging. Herein, a series of photocatalysts for aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu and Fe ions) is reported. The most active SAC-modified photocatalysts outperform not only TiO2 modified with corresponding metal oxide nanoparticles, but also the state-of-the-art benchmark photocatalysts, such as platinized TiO2 or commercial P25 powders. A combination of in-situ EPR spectroscopy and theoretical calculations revealed that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, in particular with respect to the rate of catalysis of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both effective extraction of photogenerated electrons and fast catalytic turnover in reduction of dioxygen, and translates directly into diminished recombination. These results provide essential design guidelines for the development of efficient SAC-based photocatalysts.