Photocatalytic oxidation of acetone and ethanol over nanocrystalline TiO2 powder was studied under batch conditions using an UV-illuminated DRIFTS chamber as a photoreactor. In this way, we could study the evolution of the reaction by examining changes in the species at the surface of the photocatalyst under UV irradiation. In addition, we were able to simultaneously analyze the gas-phase composition of this reaction by means of a multiple reflection FTIR gas cell. Results obtained indicate that ethanol adsorbs on the TiO2 surface either molecularly or in the form of ethoxide complexes. Under UV irradiation, these species are progressively removed, and acetate and formate complexes slowly accumulate on the TiO2 surface. Acetaldehyde builds up in the gas phase during the photocatalytic oxidation of ethanol, although this molecule is scarcely found on the TiO2 surface. The spectroscopic data provided are consistent with the existence of two parallel reaction pathways for the photocatalytic oxidation of ethanol: one yielding acetaldehyde, vapor and the other leading to CO2 through adsorbed ethoxide species. On the other hand, acetone is adsorbed exclusively in a molecular form on TiO2. Its photocatalytic oxidation yields acetate and formate complexes, along with adsorbed acetaldehyde and formic acid. These adsorbed molecules can act as intermediates species in the photooxidation of acetone. In addition, participation of specific hydroxyls groups on the TiO2 surface during this photocatalytic reaction can also be observed using the DRIFTS system. (C) 2003 Elsevier Inc. All fights reserved.