In this study, evolution and final state of particle size distribution (PSD) of TiO2/water suspension were described by a modified population balance equation considering both natural convection and Brownian motion. The calculation was verified by experimental measurement of scanning electron microscope and Laser particle analyzer. Based on the predicted PSD, Rayleigh scattering theory (for d < 4 lambda)) and Fraunhofer diffraction theory (for d > 4 lambda) were then employed to determine the scattering and absorption coefficients of TiO2/water suspensions with various particle concentrations and at various light wavelengths. UV-Vis investigation showed that the predicted optical properties of TiO2/water suspension considering PSD are in much better agreement with the experimental results than that without considering PSD, at all particle concentrations investigated. It is traditionally accepted that for colloid suspension the scattering coefficient can be neglected compared to the absorption coefficient and therefore the extinction, which is the combination of the absorption and scattering coefficients, can be approximately replaced by the absorption coefficient. However, our finding indicates that, when particle aggregation is considered, the scattering coefficient of TiO2/water suspension could be much higher than the absorption coefficient. Consequently, using the absorption coefficient alone to replace the extinction coefficient even at long wavelengths could result in significant calculation errors and is not recommended. (C) 2015 Elsevier Ltd. All rights reserved.