Reactive oxygen species (ROS) are key oxidants for the degradation of organic pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show that graphitic carbon nitride (C3N4, a famous visible-light-responsive photocatalyst) is chemically stable toward ozone and superoxide radical; in contrast, hydroxyl radical ((OH)-O-center dot) can tear the heptazine unit directly from C3N4 to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured C3N4 and bulk C3N4 that finally exists in the form of NO3- reach 9.5 and 6.8 mol % in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic ozonation, which can rapidly generate abundant (OH)-O-center dot to attack C3N4. On a positive note, in the presence of organic pollutants which compete against C3N4 for (OH)-O-center dot, the C3N4 decomposition has been completely or partially blocked; therefore, the stability of C3N4 under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of C3N4 toward (OH)-O-center dot and calls for attention to the potential deactivation and secondary pollution of catalysts in (OH)-O-center dot involved water treatment processes.