We describe the polymer templating of inorganic sol-gel precursors for making thermally stable mesoporous anatase-rutile TiO2:Ta nanocomposite films with high surface area (S-BET >= 150 m(2) g(-1)). Mesostructured films with cubic pore symmetry and pore sizes ranging from 17 nm to 26 nm in diameter were prepared from different diblock copolymers such as polyisobutylene-block-poly( ethylene oxide) and were characterized by various microscopy techniques, grazing incidence small-angle X-ray scattering, X-ray diffraction and Rietveld refinement, X-ray photoelectron spectroscopy, and impedance spectroscopy. Here, we specifically focus on the material with the smaller pores. Our research data establish that (1) the nano scale structure can be preserved up to 900 degrees C, (2) anatase is the primary phase, (3) the phase composition remains constant over a broad temperature range, (4) anatase-to-rutile transformation and formation of L-Ta2O5 do not occur below 800 degrees C, and that (5) the films contain a non-negligible fraction of Ti3+ species. The latter results in a room-temperature electrical conductivity (similar to 2.4 mS cm(-1)) much higher than that reported for stoichiometric TiO2. Compositionally, the material can be described by the chemical formula Ta0.24Ti0.75O2 +/-delta. The mesoporous nanocomposite films (E-g approximate to 3.4 eV) also display interesting photocatalytic properties and photoelectrochemical activity which makes them interesting for different applications such as wastewater detoxification. (C) 2013 Elsevier B.V. All rights reserved.