Display omitted •A new method for investigating the role of ROS during VOCs removal was developed.••OH radicals played a key free radical in o-xylene and styrene elimination.••O2− species acted as the dominant active species during acetaldehyde decomposition.••OH radicals played a leading role in the ring-opening process of benzene ring.••O2− played a crucial role in the formation of acids during acetaldehyde degradation. To identify the distinctive role of reactive oxygen species (ROS) and trace the intermediates not only help to decompose the pollutants in high efficiency but also for avoiding more harmful intermediates formed. Here, we developed a new method to generate different ROSs by controlling the atmosphere to distinguish their role in the degradation of flowing gas-phase VOCs, including o-xylene, styrene, and acetaldehyde. This method is in good agreement with the traditional sacrificial agent capture experiment. The results show that •OH radicals play a dominant role in the degradation of o-xylene and styrene, while •O2− radicals primarily take part in the acetaldehyde degradation. Additionally, we distinguish the role of holes, •OH and •O2− played during the VOCs photo-oxidation through the radical trapping, in situ DRIFTS, and GC-MS analysis. Under the attack of •O2− radicals, aromatic VOCs were photo-oxidized to intermediates containing benzene rings and ketones (i.e., toluene and butanone), while carbon chain compounds (i.e., 3-methylfuran and ethanol) tend to form under the action of •OH and holes. This can be associated with the different reaction paths initiated by ROS. For acetaldehyde removal, •O2− species facilitate the formation of acids (i.e., acetic acid) while the •OH species and holes lead to the production of ketones (i.e., acetone). This work provides deep understanding on the role of various ROS in the photocatalytic oxidation of VOCs, which can guide the design of efficient photocatalysts, selective formation of intermediates to be easily decomposed or as raw materials for further application.