A theoretical model was established to discuss the interfacial electron transfer in semiconductor heterogeneous photocatalysis. Two possible electron transfer mechanisms were compared: one is that electrons are firstly trapped by surface defects, and then transfer to O-2 (T-T transfer); another is that electrons directly transfer to O-2 from conduction band (CB) (CB-O-2 transfer). The effects of O-2 diffusion, density of electron traps, speed of electron trapping, and speed of interfacial electron transfer on the photocatalysis were studied. Based on the theoretical calculation, the most probable electron transfer was proposed. It was shown that the filling of traps by electrons plays an important role in the T-T electron transfer. For low light intensity, the quantum yield (QY) of photocatalysis is almost independent on light intensity as the electron traps are almost empty; for high light intensity, the QY decreases sharply to zero as all of traps are filled by electrons. The T-T transfer does not accord to experimental results, while the CB-O-2 electron transfer is reasonable. However, the electron trapping by defects must be important as defects are prevalent in photocatalysts, so we proposed that the most probable interfacial electron transfer is dominated by CB-O-2 transfer, but limited by trapping-detrapping through defects. (C) 2013 The Electrochemical Society. All rights reserved.