Tests have been conducted to determine the extent of dispersed oil sorption on sediments and retention of this association when seawater is flushed through the substrate. Sediment beds were prepared where dispersed oil in seawater was allowed to percolate down through the sand. Water concentrations of dispersed oil were determined before and after percolation. Distribution of oil in sediments was shown to be very patchy and not suitable for quantitating effects on benthic species. Eighty-three percent of the oil remained in the top 3 cm during this process. Additional tests in special cores showed that sediments containing dispersed oil would release about 40% of the oil when seawater flowed from the bottom of the core up through the bed of sand. Based on the fact that most of the dispersed oil is held in the top 3 cm of sub-strate when water is drained completely through the sediment, it is recommended that exposure systems be designed with a layer (2 to 3 cm) of contaminated substrate on top of clean sediment. Fiberglass trays (with mesh bottoms) were prepared in this manner and exposures initiated in both subtidal and intertidal areas in Sequim Bay, Washington. The alterations in hydrocarbon component composition during the exposure periods will be described. Biological responses measured in these studies were the growth of small clams and the recruitment of multiple benthic species.