Abstract A new transmission FTIR set-up has been developed to investigate photocatalytic reactions under in situ UV-Vis illumination and in controlled atmosphere. This system was employed to study the evolution of the surface species during the photodegradation of phenol and oxalic acid on TiO₂ P25. As far as phenol is concerned, dosage from the gas phase on the activated oxide surface resulted in dissociative adsorption for coverages below the monolayer and in physisorption beyond this limit. Experiments under UV irradiation showed no relevant photocatalytic activity in absence of O₂, the only spectroscopic consequence being the increase of a monotonic IR absorption, covering nearly the whole MIR range, related to UV-generated free conduction electrons. Only the contemporary addition of O₂ and H₂O to the TiO₂/phenol system allowed us to observe an efficient photodegradation reaction and to highlight the IR manifestations of some of the photodegradation products which are formed. In particular, oxalic acid appeared to be the main intermediate product adsorbed at the oxide surface during phenol photodegradation. For this reason the photoreactivity of pure oxalic acid on TiO₂ under UV irradiation was also investigated.