The selectivity of molecular oxygen activation on the exfoliated graphitic carbon nitride (g-C 3 N 4 ) and its influence on the photocatalytic phenol degradation process were demonstrated. Compared with bulk g-C 3 N 4 , the exfoliated nanosheet yielded a 3-fold enhancement in photocatalytic phenol degradation. ROS trapping experiments demonstrated that although the direct hole oxidation was mainly responsible for phenol photodegradation on both g-C 3 N 4 catalysts, molecular oxygen activation processes on their surface greatly influenced the whole phenol degradation efficiency. Reactive oxygen species and Raman spectroscopy measurements revealed that oxygen was preferentially reduced to ·O 2 − by one-electron transfer on bulk g-C 3 N 4 , while on g-C 3 N 4 nanosheet the production of H 2 O 2 via a two-electron transfer process was favored due to the rapid formation of surface-stabilized 1,4-endoperoxide. The latter process not only promotes the separation of photogenerated electron−hole pairs but also greatly facilitates reactive oxygen species formation and subsequently enhances phenol degradation.