Low pressure, hollow fiber (LPHF) membranes are subject to fouling by natural organic matter (NOM) during the filtration of natural waters. The mechanisms of fouling were explored in an AWWARF-funded study at bench scale using representative types of LPHF membranes and natural water samples, coupled with complimentary techniques for foulant characterization. The fouling results were quantified using the new concept called the "unified modified fouling index (UMFI)". It was found that membrane fouling is affected by multiple factors, including the presence of the high molecular weight (HMW) fraction of NOM. The magnitude of fouling was more specific to each membrane/water combination, than hydrodynamic conditions of filtration; The analysis of UMFI suggested that the observed fouling usually followed a pseudo-cake formation type of fouling, with the indices varying between 0.0027∼0.034 m2/L. Hydraulic backwashing was shown to effectively remove fouling if the proper membranes were chosen for a specific source water. The results from this study offer the membrane end user a more efficacious manner to evaluate the potential of their water to foul specific membranes by employing the developed UFMI. Moreover, it suggests important factors that may influence the fouling of their membranes by natural organic matter.