Sorption, insolubility, and related processes are largely responsible for controlling the bioavailability of many pollutants to microorganisms in soils and sediments. The turnover rates of forms of a chemical that are not bioavailable are usually slower than those for the same chemical form in solution. In addition, for many chemicals with limited bioavailabilities, the rate-limiting step in their biodegradation is mass transfer from an unavailable to available form. Consequently, because multiple processes are involved, prediction of these chemicals' biodegradation rates is far more complex than is the case for readily available chemicals. The practical issues associated with the reduced bioavailability of pollutants are many. How strongly a pollutant is sorbed, as indicated by its sorption partition coefficient, and how rapidly it biodegrades are criteria used for screening its potential to leach to groundwater. Some researchers challenge whether a strongly sorbed pollutant poses a significant environmental problem if the pollutant is not available for uptake by sensitive populations, even if its limited bioavailability also means it cannot be taken up by microorganisms that can potentially degrade it. Much of the debate over this issue is fueled by uncertainty in our understanding of coupled sorption/desorption and biodegradation processes and of their impact on bioavailability. The objectives of this chapter are to demonstrate the importance of physical mass transfer processes—including non-equilibrium sorption, diffusion and other pollutant–soil interactions—in governing rates of biodegradation of organic pollutants in soil.