The presence of fecal coliform bacteria in many watersheds is often linked to septic system effluent. Fluorescent whitening agents (FWA) are part of most laundry detergent formulations and previously have been used for fingerprinting anthropogenic sewage waters. However, the use of this fingerprinting approach is often limited by (I) below detection limit concentrations of the FWA and (II) interference with naturally occurring organic compounds. This study was carried out to develop a method that will allow, ultimately, for quantifying FWA fluxes in a small forest watershed. We conducted a series of isotherm experiments using polysulfone membranes and two types of commercially available FWA's. The goal of these laboratory studies was to quantify the uptake of the FWA by the membranes. Further, a method had to be developed to recover the FWA from the membrane filters. Quantification of the amount of FWA sorbed by the membrane is essential for studying the FWA flux in a field setting. Based on these results, a strategy will be developed for field implementation of a membrane based FWA sampler.