Poly (3-methylthiophene) (P 3 MeT) films were electrochemically deposited on titanium oxide (TiO_X) layers prepared by anodic oxidation of titanium plates, and the chemical and electronic interaction at the P 3 MeT/TiO_X 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 TiO_X 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 TiO_X surface are linked to the P 3 MeT and increase in number with decreasingthe anodic oxidation potential. This model was experimentally supported by the contact potential difference measurements of the TiO_X films.