Investiation of TiO2 aqueous suspensions has shown that their optical spectra can be unstable, with instability not related to precipitation or adherence of TiO2 particles to the vessel walls. Increase of ionic strength of the suspension as well as neutralization of charged TiO2 particles via pH adjustment accelerates the optical density drop. Vice versa, increasing the charge of TiO2 particles via shifting pH in acidic or basic directions stabilizes the suspension's optical spectra, and ultrasonic treatment promotes optical density recovery. The observed behavior is attributed to alteration in the size of the suspension aggregates. (C) 2003 Elsevier Inc. All rights reserved.