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Reactive oxygen species (ROS) play an essential role in TiO 2 photocatalysis. They arise from the transfer of lightinitiated carriers to the TiO 2 surface and react with oxygen or water, in which the TiO 2 surface is crucial. However, how the TiO 2 surface affects ROS production is unclear. Herein, dynamic generation of ROS in suspensions of TiO 2 of different particle sizes was investigated under ultraviolet-light irradiation. It is surprising to find that more ROS were produced more quickly for 100−140 nm TiO 2 than for 20−60 nm TiO 2 . Further research suggested that ROS production was intrinsically correlated with the surface bridging hydroxyls per unit area. More bridging hydroxyls induced lower IEP and more negative charges on the TiO 2 surface, which favored the transfer of photogenerated carriers, resulting in the promotion of ROS and photocatalytic activity. This provided insight into designing high-efficiency photocatalysts to solve the problem of small particle sizes causing loss and blockage in wastewater treatment.