Unmodified and sulphate-modified TiO2 samples of different crystal forms and crystal sizes were prepared at different pH, varying the source and concentration of sulphate ion and by activating at different temperatures. The samples were characterised by XRD, FF-IR, TEM and nitrogen adsorption-desorption methods. The TEM and XRD patterns revealed that TiO2 sample prepared at lower pH (pH 3) exhibit smaller crystallite size than that of higher pH (pH 9). Sulphate ion impregnation also decreases the crystallite size and stabilises the anatase phase of TiO2 at higher activation temperatures. FT-IR results show that sulphate species strongly bound bidentately on TiO2 surface. Surface area is more in case of sample prepared at lower pH than that of higher pH and also found to increase with the increase in sulphate loading up to 7.5 wt.% and thereafter decreases drastically. The photocatalytic activities of the catalysts were compared by performing degradation of 4-nitrophenol as a probe reaction. Photocatalytic degradation of 4-nitrophenol depends on surface area, the crystal form and particle size of TiO2. Samples containing mixture of anatase and rutile phases exhibit highest activity for degradation of 4-nitrophenol than that of other samples. (C) 2002 Elsevier Science B.V. All rights reserved.