Since Pt cocatalysts play an important role in photocatalytic H₂ evolution, it is necessary to track Pt over Pt/g-C₃N₄ catalysts during the evolution process to understand the associated photocatalytic deactivation behavior. In this study, bulk g-C₃N₄ (CN) and oxidized g-C₃N₄ (OCN) catalysts containing a Pt cocatalyst were prepared to investigate photocatalytic deactivation behavior through tracking changes in the catalytic properties of the Pt cocatalyst and g-C₃N₄ surface during photocatalytic H₂ evolution. While CN catalysts show a lower photocatalytic activity than OCN catalysts, the former exhibit high resistance to catalytic deactivation with a lower deactivation rate than the latter. The high photocatalytic activity of OCN catalysts is caused by the highly dispersed Pt species on chemically oxidized g-C₃N₄ with abundant O-containing functional groups, relating to the excellent separation efficiency of photogenerated electron/hole pairs. During the evolution process, highly dispersed Pt species over fresh OCN are easily and rapidly agglomerated into large Pt nanoclusters due to its exfoliated thin-layered g-C₃N₄ structure, whereas the three-dimensional multi-layered g-C₃N₄ structure of CN catalysts hinders the agglomeration of Pt over the CN catalyst. In addition, during the photocatalytic H₂ evolution, the O-containing functional groups on the OCN catalyst significantly disappear, which causes a weak metal/support interaction and, eventually, fast photocatalytic deactivation due to the agglomeration of Pt.