The rapid intermediate species accumulation on TiO₂ induces the deactivation and is a long-standing unsolved issue in the photocatalytic oxidation of volatile organic compounds (VOCs). Exploring efficient pathway to achieve deep oxidation is of crucial significance. In this study, we observed that TiO₂ with small particle size (S-TiO₂) had high toluene conversion but low CO₂ mineralization, while TiO₂ with large particle size (L-TiO₂) had low toluene conversion but high CO₂ mineralization. The characterization results showed that S-TiO₂ had a larger specific surface area and more surface hydroxyl groups than L-TiO₂, which accounted for its higher toluene conversion. Additionally, low toluene adsorption but a larger quantity of reactive oxygen species (ROS) was observed on L-TiO₂. Our findings further revealed that toluene on S-TiO₂ was initially oxidized to benzene-ring-containing intermediate species by ROS. However, the accumulation of these species suppressed the ROS production and blocked the ring-opening reaction, resulting in low CO₂ selectivity. In contrast, nearly no intermediate species was accumulated on L-TiO₂ due to the low toluene adsorption and abundant ROS production, which allowed the photocatalytic oxidation of toluene on L-TiO₂ to follow the direct ring-opening pathway and achieve higher CO₂ selectivity. This study provides new insight into the mechanism of the photocatalytic oxidation of VOCs.