TiO2 thin film photocatalysts coated onto glass plates were prepared by thermal decomposition of tetraisopropyl orthotitanate with a dip coating process using a-terpineol as a highly viscous solvent. Two types of ligands, polyethylene glycol 600 and (ethoxyethoxy)ethanol were added to the dip-coating solution as the stabilizer of titanium alkoxide. TiO2 thin films were obtained after calcination at 450 degreesC for 1 h. The film thickness obtained with a single dipping was proportional to the viscosity of the dip coating solutions. The thin films obtained were transparent with a thickness of 1 mum. The crystal form of the films was anatase alone. The thin films were formed with aggregated nano-sized TiO2 single crystals (7-15 nm), and the size of TiO2 crystals became smaller for the polymer-added systems. The photocatalytic activity of the TiO2 thin films, as evaluated by the photooxidation of NO (1 ppm) in dry air, was as high as our previous TiO2 thin films prepared by the sol-gel method.