Methods for collecting interstitial water (IW) in laboratory ecotoxicological testing were compared and used to characterize the partitioning behavior of a cationic surfactant in batch sediment/water systems. IW sampling methods included a conventional centrifugation technique, dialysis through a rayon membrane, and a novel fritted glass diffusion sampler. Dodecyl trimethyl ammonium chloride (TMAC) was the test compound monitored radiometrically in each static-renewal experiment. A 10-mg/L TMAC solution was delivered to test systems as overlying water, replenished daily; TMAC levels in sediment, interestitial water, and overlying water were monitored throughout the 20-day experiments. IW samples collected by the centrifugation procedure contained greater TMAC concentrations than samples collected by either dialysis or the fritted glass sampler. Equilibration of TMAC concentrations across the dialysis membrane was slow (>5 days), resulting in a substantial lag. Advantages of using the fritted glass sampler for ecotoxicological testing are discussed. Following rapid changes in TMAC levels in each compartment during the initial five days of each experiment, order of magnitude differences were observed in TMAC concentrations among sediments, overlying water, and interstitial waters, in decreasing order. The partition coefficient measured in separate shake-flask experiments performed at a high solids concentration was similar to the observed ratio between TMAC levels in sediments and interstitial water. Sorption of TMAC onto sediments was consistent with a first-order kinetics process.