Transparent nanophase TiO2 thin films on soda lime glass were prepared from titanium tetraisopropoxide (TTIP) by a sol-gel dip-coating method. The TiO2 films had amorphous phase up to 400degreesC and anatase phase at 500degreesC. The amorphous TiO2 films obtained at 300-400degreesC showed considerable photoactivity for the degradation of formic acid. The photoactivity of the TiO 2 films was enhanced with increasing calcination temperature from 300degrees to 500degreesC. The crystallinity of the anatase films at 500degrees was improved with increasing calcination time up to 2 h and reduced with a further increase in calcination time to 4 h due to the significant formation of sodium titanate phase as a result of sodium diffusion. The four-time-dipping anatase films at 500degreesC exhibited the greatest photoactivity at the calcination time of 2 h. Sodium diffusion into TiO2 films was retarded by a SiO2 underlayer of 50 nm in thickness.