Molecular weight distributions of Dissolved Organic Matter in natural waters (Croton Watershed and Onondaga Lake (Syracuse)) were studied using gel permeation chromatography (GPC). In both waters the UV absorbance intensity increased with depth while the Dissolved Organic Carbon characteristics showed little or no change; photo bleaching has been hypothesized to be the cause of this trend. This was investigated further by exposing water to sunlight on a roof top and analyzing the DOC characteristics as well as the trihalomethane formation potential. The trihalomethane formation potential was found to decrease with time. The trihalomethane formation over an eight week period without exposure to sunlight showed little variation, indicating that exposure to sunlight affects trihalomethane formation potential. Sun incidence data was also used to compare with trihalomethane formation data. The results indicate that greater sunlight incidence led to less trihalomethane formation.