The generation of hydroxyl (•OH) radicals plays a key role in the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO2. The quantum yield of this process is thus an important parameter; however, it is not easy to measure in a particulate system arising from problems caused by light scattering from the particles. In this work, a reliable method for the determination of the quantum yield of hydroxyl radical production in heterogeneous systems has been developed, based on measurements of •OH radical generation rates and the photon flux absorbed by TiO2 suspensions. In this procedure, a modified integrating sphere method was used to determine the true fraction of light absorbed by TiO2 suspensions. A ferrioxalate chemical actinometer was used to measure the incident photon flux. As a check on the quantum yield method, good agreement with known literature values was obtained for quantum yield measurements of the photochemical generation of the p-benzosemiquinone (BQ•-) radical in the photolysis of p-benzoquinone and of the •OH radical generation in the photolysis of hydrogen peroxide, respectively. Accordingly, the quantum yield for the •OH radical production in the TiO2 suspension was determined to be 0.040 ± 0.003 at pH 7. Effects on the quantum yield of suspension loading, photon flux, and electron-acceptor addition (H2O2 and O2) were explored.