Thin films of 2,2'-bithiophene (2T) and 2,2'-5',2"-terthiophene (3T) are polymerized by UV irradiation. The polymerization process can be observed by UV-vis and photoluminescence spectroscopy. If 2T is used as starting material, predominantly 4T (quarterthiophene) is formed, while 6T (sexithiophene) is formed if 3T is used as starting material. The same polymerization reaction occurs if solutions of 2T or 3T in chloroform or toluene are used or if a film of nanocrystalline titanium dioxide (nc-TiO2) is soaked with a 2T or a 3T solution. In the latter case the pores are (partly) filled with 4T or 6T, which acts as hole conductor. A photovoltaic response is observed when nc-TiO2/4T or nc-TiO2/6T interpenetrating structures are irradiated with visible light. The increase of oligomer length from 2T to 3T and the corresponding decrease in orientational freedom of the molecules strongly affects the ease of polymerization and the structural order of the compounds formed.