In this work, the photocatalytic generation of free OH radicals ((OHfree)-O-center dot) in aqueous TiO2 suspensions was studied using an O-18 isotope labeling and a "remote" photocatalysis approach. A probe compound, 1,3,5-trichlorobenzene (TCB), was adsorbed in pores of silica gel (SG) microparticles and, by this, was shielded from the direct hole oxidation. Penetration of the TiO2 particles (25 nm in size) to the TCB, adsorbed in the SG pores, was blocked due to a small size of the SG pores (4 nm). Therefore, the "remote" degradation of the TCB was solely driven by (OHfree)-O-center dot, and this allowed us to selectively determine the quantum yield of the (OHfree)-O-center dot generation. The isotope labeling with O-18 has demonstrated that the major pathway of the (OHfree)-O-center dot formation is the direct hole oxidation of H2O, whereas the reduction of dissolved O-2 by photogenerated electrons contributes in less than 5% of the total amount of (OHfree)-O-center dot. Nevertheless, the latter pathway becomes more important if holes are scavenged. This work sheds light on the intrinsic roles of photogenerated holes and electrons in the mechanism of the (OHfree)-O-center dot formation in aqueous TiO2 photocatalysis. (C) 2015 Elsevier B.V. All rights reserved.