Sorption of hydrophobic organic chemicals in natural sediment suspensions was found to frequently require extended time periods (days to weeks) for complete equilibration. Sorption dynamics could be described by a two-compartment model that distinguished rapid or “labile” exchange (requiring at most a few hours to achieve) from highly retarded or “nonlabile” sorption requiring days to weeks to occur. In general, one-half or less of the total sorption was labile. 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. That is, the more highly sorbed chemicals sorbed more slowly. For some sediments, air-drying to facilitate sample transport or storage was found to result in formation of highly stable aggregates that severely altered sorbent availability to hydrophobic chemicals. An understanding of sorption dynamics is important in describing the fate of highly sorbed pollutants in aquatic systems.