Dissolved organic matter (DOM) leached from plastics could be accelerated when plastics lose their protective additives during their transport and weathering processes in aquatic systems. In this study, we used two additive-free plastics (polyethylene and polypropylene) to study its leaching potential in artificial freshwater under UV radiation and dark conditions. Its potential to generate trihalomethanes (THMs), a well-known environmental impact of natural DOM, was also explored. We found that under UV irradiation, ∼3% of the total mass of the polymer was released to the aquatic media as DOM. Unlike natural DOM, microplastic DOM was mostly composed of low molecular weight fractions < 350 Da. The DOM released by both polymers exhibited a high potential to form THMs upon chlorination, which was comparable to those of typical aquatic natural DOM. This study highlights the potential risk of plastic DOM to produce toxic disinfection byproducts (DBPs) upon chlorination in wastewater plants.