Poly (3-methylthiophene) (P 3 MeT) films were electrochemically deposited on titanium oxide (TiOX) layers prepared by anodic oxidation of titanium plates, and the chemical and electronic interaction at the P 3 MeT／TiOX p-n heterojunction interfaces were investigated by current-voltage and capacitance-voltage measurements. The junction properties (diode factor, rectifying ratio, barrier height and built-in potential) of the cells were spontaneously improved during their storage in the air, and the extent of the improvement was higher when the TiOX was prepared at a lower one (5 V) than in the case of a higher oxidation bias (20 V). These experimental findings were well explained by using a junction formation model described as the evolution of covalent bond formation at the interface, in which chemically active species on the TiOX surface are linked to the P 3 MeT and increase in number with decreasing the anodic oxidation potential. This model was experimentally supported by the contact potential difference measurements of the TiOX films.