Mesoporous TiO2 films were synthesized by using triblock copolymers via a sol-gel process in aqueous solution. It was found that a film calcined at 600 degrees C has the highest photocatalytic activity. By application of positron annihilation Doppler broadening spectroscopy combined with XRD, SEM, and N-2 adsorption desorption techniques, the film structural properties were examined systematically. It is revealed that an excellent photocatalytic activity could be achieved only if a film maintains a suitable TiO2 grain size, i.e. similar to 7.5 nm in the present study, a median specific surface area and a high crystallinity in anatase state. (C) 2010 Elsevier B.V. All rights reserved.