The chemical release rates from laboratory and field contaminated sediments were studied. Contrary to reports that sorption rates are inversely related to Kow , the slow desorption rates were found to be similar for different compounds. The data were modeled by a two compartment irreversible adsorption and radial diffusion model. Desorption kinetics from the first irreversible compartment can be modeled by radial diffusion and assume an irreversible adsorption constant and soil tortuosity of 4.3. The desorption half life is approximately 2-7 days. Desorption from the second irreversible compartment is very slow, (half-life of approximately 0.32 -8.62 years) presumably caused by entrapment in soil organic matter that increases the constrictivity of the solid phase to chemical diffusion. From the kinetic data, it is deduced that the diffusion pore diameter of the second irreversible compartment is approximately equal to the critical molecular diameter. The mass of chemicals in this highly constrictive irreversible compartment is approximately one fourth of the maximum irreversible, or resistant, compartment. The slow kinetics observed in this study add additional support to the notion that the irreversibly sorbed chemicals are "benign" to the environment.