Sorption of hydrophobic organic chemicals in natural sediment suspensions was found to frequently require extended time periods (days to weeks) for complete equilibrium. Sorption dynamics could be described by a two-compartment model that distinguished a rapid or 'labile' exchange(requiring at most a few hours to achieve) from highly retarded or 'nonlabile' sorption requiring days or weeks to occur. For highly hydrophobic chemicals and high solid concentrations, the labile fraction decreased to 0.1 or less in some systems. The kinetic exchange constant for nonlabile sorption varied inversely with the sorption equilibrium constant. An understanding of sorption dynamics is important in describing the fate of highly sorbed pollutants in aquatic systems.