Nanocrystalline TiO2 is known to be a very efficient photocatalyst. In order to elucidate the details of reaction pathways occurring on the surface, nanocrystalline TiO2 films (With 12 run average crystallite size) were covered by methylene blue (NIB) and studied, both in the pristine state and upon UV exposure, by TOF-SIMS. Distinct mass signals related to the MB parent molecule (m = 284.1 amu) and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is pronouncedly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the amount of the parent molecule is diminished whereas intermediate reaction products are identified to be present at the TiO2] surfaces. Eventually, the parent molecule and the characteristic fragment species disappear completely upon extended exposure to UV light. (c) 2006 Published by Elsevier B.V.