Natural organic matter (NOM) in natural water is responsible for significant organic fouling of low pressure membranes. Membrane fouling can be affected by the characteristics of the NOM and its interaction with the membranes and their associated properties. Three different water sources were tested for this study; river water, wastewater treatment plant secondary effluent and water with high content of DOC and humic substances. Three different types of low pressure flat sheet membranes (MF and UF) were selected which are equivalent simulation of commercial hollow fiber membranes, and dead-end, stirred cell filtrations were performed. From the water source characterization, Scottsdale WWTP secondary effluent has high content of high molecular weight compounds corresponding to colloids and/or macromolecules by SEC-DOC/UV and showed significant flux decline in all of membrane filtration. Tampa water contain relatively less high molecular weight compounds and showed significant flux decline comparing to White River water. This may be due to over three time higher DOC content than other water sources. Thus, the water containing high colloids and macromolecules as well as high DOC content is very problematical in low-pressure membrane fouling. SEC-DOC/UV spectrum of backwashed stream proved that hydraulically reversible foulants are high molecular weight compounds and those molecules are likely to cause significant flux decline by cake layer formation on membrane surface.