The use of low-pressure membranes, microfiltration (MF) and ultrafiltration (UF), has increased dramatically over the last decade in response to new drinking water regulations focused on increased control of pathogens and disinfection byproducts (DBPs), and reduced membrane costs. The overall goal of this project was to investigate the specific contributions of the different types of natural organic matter to MF / UF fouling with the intent to develop a surrogate test or index that could be used to predict NOM fouling at low cost through a combination of source water characterization and rapid bench-scale testing.