The degradation of compounds at concentrations normally encountered in environmental samples can be adequately described by a second-order rate expression, which accounts for sorption in sediment:water systems. ATP is used as a measure of population size. It will be shown that sorption of toxics to suspended sediments rendered the compound unavailable for degradation in the absorbed state. Further testing of this kinetic expression is needed, however, for broader classes of compounds having wider ranges of sorptive properties.