The initial fouling behavior of a clean membrane surface was studied using flow field-flow fractionation (flow FFF), an analytical technique typically used to separate and characterize macromolecules and particulates. This work represents the first time flow FFF has been used to quantitatively evaluate membrane performance. Flow FFF is an ideal tool for expeditiously studying sample-membrane interactions for the following reasons: membranes can be quickly installed into the flow FFF channel, each analysis requires only microgram amounts of sample, and sample-membrane interactions can be rapidly quantitated for different flowrates and solution compositions. Suwannee River humic acids were used as a probe to investigate the initial fouling of an XLE reverse osmosis membrane and an NF-200 nanofiltration membrane. Flow FFF was successfully used to quantitate the fouling of each membrane and to demonstrate that the majority of sample loss was due to irreversible adsorption. The fouling on both membranes was enhanced by increasing the flowrate perpendicular to the membrane surface and by adding calcium ions to the solution. The NF-200 membrane was more resistant than the XLE membrane to fouling in the presence of calcium ions, whereas, the fouling resistance of both membranes improved to similar levels with the addition of EDTA to a solution containing calcium ions.