Nanocrystalline TiO2 photocatalysts were synthesized in mild conditions by thermohydrolysis of TiOSO4 in water at 100 A degrees C and post-calcination treatment at various temperatures. The TiO2 powders were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, specific surface area determinations, scanning electron microscopy and electron paramagnetic resonance measurements. The photoactivity of the samples was tested employing the photodegradation of 4-nitrophenol in liquid-solid regimen and the photooxidation of gaseous 2-propanol. The best results were obtained with the powder calcined at 600 A degrees C for 10 h. Surprisingly, the not calcined sample was the most active for the abatement of NOx under irradiation.