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TiO2 thin films were prepared on quartz glass by sol-gel process and their structural and optical properties were examined at various catalyst concentrations and calcination temperatures. The as-deposited TiO2 thin films are amorphous, and they transform into the anatase phase at 400–600 °C, and into the anatase–rutile phase at 800 °C, and further into the rutile phase at 1000 °C. The temperature of phase transformation is decreased with the concentration of catalyst HCl. The crystallite size of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. The secondary particles in the TiO2 thin films grow by intra-agglomerate densification below 800 °C, whereas they grow by inter-agglomerate densification above 800 °C. The deposited TiO2 thin films have high transparency in the visible range. The transmittance of the films calcined at 800 and 1000 °C are significantly reduced in the wavelength range of 300–800 nm due to the change of crystallite phase and increased particle size. The refractive index of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. On the other hand, the porosity of TiO2 thin film is decreased. [Copyright &y& Elsevier]