The presence of algal organic matter (AOM) and emerging organic contaminants in surface drinking water sources has received increasing attention recently. Nevertheless, interactions between these two harmful co-existing compounds in water sources have rarely been reported. This study investigated the influence of AOM, including intracellular organic matter (IOM) and extracellular organic matter (EOM), on the photo-degradation of Diuron, under UV and visible light (VL) irradiation. Kinetics analyses indicated that both the direct and indirect photolysis of Diuron followed a pseudo-first-order reaction (R-2 > 0.98). Diuron was rapidly removed under UV irradiation accompanied by minor impacts from AOM. Under VL irradiation, the photo-excited phycocyanin in the IOM was capable of accelerating Diuron degradation, whereas the coexistence of other organic compounds in the IOM inhibited the reaction due to a screening effect, resulting in a slower overall rate of Diuron degradation. By comparison, EOM promoted the reactions when pH exceeded 5.0. The AM-mediated generation of triplet DOM ((DOM)-D-3*) accounted for 82% and 64% of the Diuron removal in the presence of IOM and EOM, respectively. Spectral excitation-emission matrices produced by AOM in solutions demonstrated the primary mechanisms of the co-existing AOM on Diuron degradation. (C) 2015 Elsevier B.V. All rights reserved.