A new negligible depletion extraction procedure was proposed for equilibrium sampling of 4-tert-octylphenol (OP) and 4-nonylphenol (NP) into a thin film of 1-octanol supported on a hollow fiber membrane. This thin liquid film extraction technique was directed at the determination of ( 1) freely dissolved concentrations, ( 2) distribution coefficients to 1-octanol (D-ow), and ( 3) binding to dissolved organic matter (D-DOC). The sampling device was prepared by dipping pieces of polypropylene microporous hollow fiber membrane (10-mm length, 30-mu m wall thickness, 240-mu m inner diameter) into 1-octanol for a few seconds to impregnate the pores of the hollow fiber wall. After stirring in 100 mL of sample solution for 24 h, the sampling device was harvested and desorbed with 30 mu L of methanol, of which 20 mu L was injected for HPLC analysis. With the measured Dow of a chemical and its equilibrium concentration in the 1-octanol sampling phase (C-octanol), the freely dissolved concentration (C-free) was calibrated based on C-free = C-octanol/D-ow. Measured log D-ow values of OP (4.32 +/- 0.06) and NP (4.79 +/- 0.02) were independent of the chemical concentration, only minimally affected by the environmentally relevant pH, buffering capacity, and salinity of samples, and agreed well with reported values. Log D-DOC values of OP (4.89 +/- 0.43) and NP (5.14 +/- 0.37), determined in Aldrich humic acid solution, agreed with reported partition coefficients to organic carbon ( log K-oc) for particles in river water and effluent wastewater. Short equilibration times and high enrichment factors were obtained for both analytes due to the high surface to volume ratio of the new sampler. The technique was successfully applied to determine C-free of OP and NP in real water samples and to study their association with humic acids and bovine albumin.