The primary objective of the research conducted by the Battelle/Marine Research Laboratory (MRL) for the U.S. Environmental Protection Agency was to evaluate the equilibrium partitioning (EP) theory as a candidate method that would allow this extrapolation to be made. The theory may provide the theoretical basis for understanding how complex interactions between chemicals and sediment influence toxicity of the chemicals to benthic organisms. Applying EP theory to sediment quality criteria development is based on the additional assumption that the only toxic fraction of a sediment contaminant is that fraction freely dissolved within interstitial waters. This assumption permits data from water quality criteria, or other aquatic toxicity data to be used to derive sediment quality criteria. To evaluate the accuracy of EP theory in predicting the toxicity of sediment-associated nonpolar organic contaminants, three research hypotheses were examined: (1) the concentration of the contaminant in solution in the interstitial water is the primary determinant of acute toxicity, (2) the toxic concentration can be predicted from the organic carbon/water partition coefficient and the concentration of organic carbon in the sediment, and (3) the source of the organic carbon influences the toxicity.