Photocatalytic thin films consisting of ionic polymers and positively charged TiO2 nanoparticles were fabricated by the layerby-layer self-assembling (LBL-SA) method, i.e. successive ionic adsorption from their aqueous solutions. Regular increases of the film thickness and the film weight by each deposition were characterized using ellipsometry, UV-Vis spectroscopy, and a quartz crystal microbalance QCM). Uniformly distributed TiO2 nanoparticles in the thin film were also observed in scanning and transmission electron microscopic images. Photocatalytic properties of the LBL-SA thin film containing TiO2 nanoparticles were confirmed by oxidation of iodide and decomposition of methyl orange. These results indicate a controllable LBL-SA process to immobilize photocatalytic TiO2 nanoparticles in thin films. (C) 2002 Elsevier Science B.V. All rights reserved.