Titanium(IV) oxide (TiO2) powders synthesized by HyCOM (hydrothermal crystallization in organic media) method, which had been proved to exhibit ultra-high photocatalytic activity in several reaction systems, were used as starting material for fabrication of transparent TiO2 thin films. HyCOM-TiO2 powders were dispersed in aqueous sol of nitric acid to yield a TiO2 sol stable for more than 90 days. Transparent TiO2 thin films were successfully produced by dip-coating from the TiO2 sol and used for photocatalytic decomposition of malachite green (MG) in an aqueous sol under aerated conditions. These films exhibited much higher rate of MO decomposition compared with those prepared from a commercially available TiO2 sol developed for photocatalytic use (Ishihara STS-01), indicating that the excellent photocatalytic activity of original HyCOM-TiO2 particles was preserved after immobilization on glass substrates by the present method. (C) 2001 Elsevier Science B.V. All rights reserved.