A combination of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, UV−vis spectroscopy and transmission electron microscopy was used to study the adsorption of thiol-protected gold nanoparticles on TiO2 films and the behavior of the resulting composite films upon UV irradiation. The gold nanoparticles were covered by charged thiols N-acetyl-l-cysteine and l-glutathione and had a mean core diameter of about 1 nm. The TiO2 film was prepared by deposition of a slurry of TiO2 nanoparticles with a particles size of 21 nm. The combination of the two spectroscopic techniques showed that the adsorption of the gold nanoparticles onto the TiO2 films is significantly limited by intrafilm diffusion. Upon illumination the IR spectra revealed the removal of the adsorbed thiolates and the appearance of sulfates. These species were also observed when N-acetyl-l-cysteine adsorbed on TiO2 was illuminated, i.e., in the absence of gold. In the latter case oxalate was observed in large quantity on the TiO2 surface, in contrast to the illumination of the N-acetyl-l-cysteine-protected gold particles. This indicates a different pathway for the decomposition of the adsorbed thiol when adsorbed on the gold or directly on the TiO2 surface. In situ UV−vis spectroscopy also shows the formation of larger particles upon illumination, which is confirmed by transmission electron microscopy.