Active oxygen species generated by photoexcitation of TiO2 sol solutions were directly observed by the single-shot near-field heterodyne transient grating method. Two kinds of dynamics due to chemical species were observed and their lifetimes and diffusion coefficients were determined. From the dependence of the quantity of dissolved oxygen, it was confirmed that one of the species was generated from dissolved oxygen, such as a superoxide anion. Furthermore, this species was found to have a very small diffusion coefficient compared with other molecules with similar molecular weight, thereby suggesting that it was adsorbed onto TiO2 particles. (C) 2008 Elsevier B.V. All rights reserved.