This study investigated the role of the band gap, surface area, and phase composition on the photocatalytic activity of nanocrystalline TiO2. Nanocrystalline TiO2 (8-29 nm) was synthesized by hydrolysis of titanium tetraisopropoxide. The crystalline structure, band gap, and morphology of the nanocrystalline TiO2 were determined by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and N-2 adsorption (BET) at 77 K, respectively. It is observed that the band gap of the nanocrystalline TiO2 decreases from 3.29 to 3.01 eV with increasing calcination temperature. The crystallite size of the TiO2 samples prepared also shows an increase with increasing calcination temperature. The photocatalytic degradation of an aqueous solution of nitrobenzene (50 ppm) was studied using nanocrystalline TiO2 samples with varying band-gap values, as well as a P-25 Degussa TiO2 sample for comparison. The initial rate of degradation of nitrobenzene was calculated in each case to evaluate the photocatalytic activity of the catalysts. The enhanced photocatalytic degradation of nitrobenzene was observed by purging air through the solution during photocatalysis.