The photocatalytic oxidation of diuron has been performed in presence of TiO2 suspensions. To better understand the mechanistic details of the hydroxyl radical attack on diuron, computational methods were carried out. The combination of experimental and computational methods has been employed to establish the main degradation pathways of diuron. After identification of the majority of first byproduct, the experimental study underlined that substitution of chlorines and hydroxylation by hydroxyl radicals are the main diuron degradation pathways, in agreement with MOPAC and DFT simulations carried out in the gas phase. In addition, these calculations revealed that the aromatic ring attack is led by a HO degrees addition and not a hydrogen abstraction and the main first byproduct, monochlorohydroxylated compounds, are obtained by concerted reactions.