Titanium dioxide (TiO2) has received intense interest because of the numerous technical applications. In this work, the TiO2 thin films were deposited on Si substrates by spin coating from a TiO2 solution prepared by a sol-gel method. It was found that the TiO2 film properties strongly depended on the post annealing conditions. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrometer (FTIR) were used to investigate the compositional and chemical states of the thermal-treated TiO2 samples. For the TiO2 samples annealed in atmosphere, the contents of -C and -OH bonds increased when the process temperature increased. From XPS measurement, the atomic concentrations of carbon and oxygen in the film are determined to be approximately 46% and 37%, respectively. However, for the samples annealed in oxygen, the contents of -C and -OH bonds were found to decrease when the anneal temperature increased. The concentrations of carbon and oxygen in the film were approximately 11% and 59%, respectively. To further investigate the behavior, the in-depth concentration profile of carbon was measured for the TiO2 samples under different etching time. It was found that the carbon concentration drastically decreased from 11% (surface) to 0.2% (near the interface). The result indicates that the pure TiO2 film could be obtained by sol-gel coating with a suitable annealing process.