© The Royal Society of Chemistry 2018. This study evaluated the effects and related mechanisms of natural organic matter (NOM) on the photolysis of methyltriclosan (MTCS), a metabolite of triclosan. Addition of two representative NOM isolates, Pony Lake fulvic acid (PLFA-microbial origin) and Suwannee River fulvic acid (SRFA-terrestrial origin), significantly inhibited the direct photolytic rate of MTCS by ∼70%. The MTCS photolytic rate in the presence of PLFA was greater than for SRFA. NOM not only suppressed photolysis by light-shielding, but also produced ROS to oxidatively degrade MTCS and/or triplet NOM (3NOM∗) to sensitize degradation. The dual effects of light-screening and photo-sensitization led to an overall decrease in photolysis of MTCS with a positive concentration-dependence. Upon addition of NOM, EPR documented the occurrence of 1O2 and OH in the photolytic process, and the bimolecular k value for the reaction of 1O2 with MTCS was 1.86 × 106 M-1 s-1. ROS-quenching experiments indicated that the contribution of OH (19.1-29.5%) to indirect photolysis of MTCS was lower than for 1O2 (38.3-58.7%). Experiments with D2O further demonstrated that 1O2 participated in MTCS photodegradation. Moreover, the addition of sorbic acid and O2 gas to the reaction confirmed the participation of 3NOM∗ as a key reactant in the photochemical transformation of MTCS. This is the first comprehensive analysis of NOM effects on the indirect photolysis of MTCS, which provides new insights for understanding the environmental fate of MTCS in natural environments.