Titanium dioxide nanopowders were synthesized mechanochemically using TiCl4 and (NH4)(2)CO3 (series TC) or TiCl4/(NH4)(2)CO3 and Na2SO4-Na2SO4 center dot 10H(2)O (series TCSM), as precursors and post-annealing in the temperature range 150-850 degrees C. Crystal structure and crystallite size of the titania nanomaterials were characterized by X-ray diffraction, and the optical properties of aqueous suspensions at various TiO2 loadings were measured using UV/visible spectroscopy. The photoinduced activity was measured in situ by EPR spectroscopy applying a variety of experimental techniques (spin trapping, spin probes, 4-hydroxy-2,2,6,6-tetramethylpiperidine oxidation). In the series TC the presence of an NH4Cl salt matrix, which was removed by decomposition to gaseous byproducts in the post-annealing process, leads to the formation of anatase nanocrystals with optimal size for high photoactivity (23-50 nm) at temperatures of 550 and 650 degrees C, and these titania samples are the most active in the photoinduced production of reactive oxygen species. The results found for TCSM series confirmed that the presence of a sulfate salt matrix (Na2SO4) during mechanochemical and annealing processes significantly hindered TiO2 nanocrystallite growth, increased agglomerate formation in aqueous media and lowered the values of photonic efficiencies determined via paramagnetic species formation or termination. (C) 2010 Elsevier B.V. All rights reserved.