The photocatalytic degradation of methylene blue and 4-chlorophenol on nanocrystalline TiO2 (nc-TiO2) under UV irradiation was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Nanocrystalline TiO2 films were prepared from suspensions containing TiO2-crystallites of different average sizes, the smallest one being 12 nm. The organic substances (either methylene blue or 4-chlorophenol) were applied to these films. The specimens were studied in the pristine state and upon UV exposure. The UV illuminations were carried out both under atmospheric conditions and in situ under ultrahigh vacuum in the ToF-SIMS instrument. Distinct mass signals from the parent molecules and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is significantly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signals are strongly diminished whereas fragmentation products are identified to be present at the TiO2 surfaces. UV irradiations carried out under different vacuum conditions in the ToF instrument (ultrahigh vacuum, air or oxygen adsorption) indicate that varying ambient conditions may influence the photocatalytic reaction on the nanocrystalline TiO2 films. (C) 2008 Published by Elsevier B.V.