Trap states play important role in electron transport of dye sensitized solar cells (DSSCs). Different trap states (surface and bulk traps) contribute differently to the performance of DSSCs. However, there is a lack of classification of the trap states, especially in recent doping works of the photoanodes. In this work, the Ce4+ (0.3, 0.6 and 0.9% molar ratio) in TiO2 and Ti4+ (15, 40 and 70% molar ratio) in SnO2 are assigned to surface traps and surface-and-bulk coexisted traps, respectively. The property of each trap state and its influence to the electron transport are characterized. Both the surface and bulk traps deteriorate the electron transport in DSSCs, however, the negative role of surface traps can easily be eliminated by surface modification in contrast to the bulk traps. Furthermore, contrary to the literature that the trap states will accelerate the interface recombination, it is found that the interface electron recombination time is prolonged with Ce4+ surface traps in TiO2 and Ti4+ bulk traps in SnO2, indicating that the recombination time is closely related to the property of the trap states. (C) 2014 Elsevier B.V. All rights reserved.