The effect Of O-2 doping on titanyl plithalocyanine (TiOPc) thin films was investigated by ultraviolet photoelectron spectroscopy (UPS). The results revealed a clear change in the film thickness dependence of the energy of the electronic levels on exposure to O-2. The film deposited in ultrahigh vacuum showed downward band bending characteristic of a n-type semiconductor, probably due to unintentional doping by residual impurity. On the other hand, the film deposited in O-2 atmosphere showed upward band bending characteristic of a p-type semiconductor. This conversion is ascribed to the hole doping by O-2. These results correspond well with the reported change of the electrical behavior for TiOPc-based field effect transistors. (c) 2005 Elsevier B.V. All rights reserved.