This study demonstrated the effects of supported Au nanoparticles of various sizes and shapes on its co-catalytic activity imparted to TiO2 during photocatalytic oxidation of salicylic acid. The TiO2 photoactivity is remarkably improved with the decreasing size (9.5 +/- 0.06 to 3.5 +/- 0.25 nm) and increasing surface to volume (S/V) ratio (0.629-1.95 nm(-1)) of spherical Au co-catalysts loading. The amount of Au (0.02 wt%) nanostructures supported to TiO2 for its optimum photoreactivity is found to be 100 times less than the conventional prerequisite of 1-2 wt% metal photodeposition. The Au nanorod (aspect ratio = 2.8 +/- 0.12 and S/V = 0.54 nm(-1)) attachment to TiO2 significantly decreased the photoactivity compared with the highly active quantum size (3.5 +/- 0.25 nm) Au co-catalysts loading. The interaction of Au nanoparticles of various morphology with TiO2 induces the photoexcited charge transfer process in varied extent, leading to diverse photocatalytic activity. Zeta potential (surface charge and conductance) measurement of aqueous dispersion of TiO2. Au nanoparticles and salicylic acid was carried out to investigate the interaction among the various components in the photoreaction system. (C) 2011 Elsevier B.V. All rights reserved.