Sorption and desorption processes strongly affect fate, transport, bioavailabilty, and risks of organic contaminants in the environment. Conventional isotherms often fail to represent precisely the release of sorbed contaminants, because desorption is often not the simple reverse of sorption. This phenomenon is referred to as “irreversible sorption” and is most likely caused by the physical entrapment of a fraction of sorbed chemicals in soils and sediments, according to the Irreversible Sorption Mechanism proposed by the authors. A newly developed Irreversible Sorption Isotherm quantifies the desorption of reversibly and irreversibly sorbed chemicals with different mathematical functions and thus, can better model the long-term resistant release of hydrophobic organic compounds. The impact of irreversible sorption is twofold. Irreversible sorption may cause more contaminants to stay in the sorbed phase and hence, may keep contaminant plumes from migrating. However, it could also enhance the difficulties of soil/groundwater remediation. Irreversible sorption could reduce the bioavailablity of sorbed contaminants. However, the same process could also reduce the ecological and human-health risks of sorbed contaminants. A simple solute transport model – ISAST – was developed to assess the impact of irreversible sorption on contaminant transport in subsurface and on soil/groundwater remediation.