Polypropylene (PP) and polyethylene (PE) are commonly used polyolefins in a variety of applications, which have resulted in their accumulation in the environment. Once in the environment, these polymers undergo various chemical and physical transformations as the result of environmental stressors such as sunlight. During photodegradation, PP and PE undergo reactions categorized as oxidation, crosslinking, and chain scission that are induced from UV light; yet, there are key gaps in knowledge on the phototransformations that occur under aqueous conditions. Therefore, it is the goal of this project to characterize the phototransformations of PP and PE in simulated natural water conditions. This thesis focuses on 0.03 mm and 0.025 mm thick PE and PP thin films respectively. The polymer films were irradiated with 254 nm and 350 nm UV light in air, ultra-pure water, and solutions of dissolved organic matter (DOM) (10 mgC/L Suwanee River natural organic matter) to simulate natural systems. For comparison, the films were subjected to natural weathering over the course of Summer 2019 in Duluth, Minnesota. Irradiated plastics were then evaluated for a variety of chemical transformations. It was observed using Fourier Transform Infrared Spectroscopy (FTIR) that oxidation occurred both in air and aqueous environments, with oxidation in aqueous environments happening at a slower rate. However, in the presence of DOM, indirect photochemistry accelerated oxidation compared to ultrapure water, providing the first evidence of indirect photo-transformations of plastics. Polymer crystallinity, measured with FTIR, was also monitored as an evaluation of polymer scission. An increase in crystallinity was observed for all samples indicating that the polymer matrix was rearranged during photodegradation, with samples in water showing a sharper increase in crystallinity initially. Compared to naturally weathered sample, lab observed transformations were in line with natural sampling supporting the importance of the lab-based measurements. Through this work, we have gained a clearer perspective on the chemical weathering of materials found in aquatic plastic debris, which will allow us to predict the behavior of these materials, including the breakdown into microplastics.