In aqueous photoreaction systems, the existence of gradient light intensity throughout the reaction solution is one of the important restrictions to the efficiency of the overall photoreaction, which highly relies on the optical properties of the catalyst such as light absorption and scattering effects, the particle size of the catalyst, and the degree of agglomeration in its slurry. In this study, both synthesized TiO2 hydrosol and commercial TiO2 powder, Degussa TiO2 P25, were used as catalysts for comparison, in which their particulate and optical properties in aqueous solution/suspension Were investigated The characteristic results showed that the TiO2 hydrosol had smaller crystal sizes and particle sizes, but a lower degree Of crystallinity compared to the P25 powder. As TiO2 hydrosol solution had the much lower light extinction coefficient than P25 suspension, the attenuation of light intensity gradient in the reaction solution containing more transparent TiO2 hydrosol was greatly eliminated. However, both types of catalysts demonstrated similar effects of significant light scattering in their solution/suspension. The photocatalytic activities of two catalysts were then evaluated in two sets of experiments for methyl orange (MO) and methylsulfonic acid (MSA) degradations, respectively. The much higher reaction rates of MO and MSA degradations were found in the TiO2 hydrosol solution than in the P25 suspension, because TiO2 hydrosol catalyst with much finer particles and higher transparency allowed the photoreactor system to work at a higher catalyst loading than the conventional TiO2 powdery catalyst A new kinetic model considering three main factors of (i) the volumetric rate of photon absorption, (ii) effective reaction volume, and (iii) the effective surface area of catalyst in a cylindrical photoreactor was also established. This new model summarizes that the catalyst with a higher surface area, a lower light extinction coefficient, and a lesser light scattering effect would achieve better light utilization within the photoreactor system and is beneficial to achieve a higher efficiency of overall photoreaction.