The importance of chemical conditions and mass transfer effects to in situ bioremediation of PAHs is presented using a case study. In situ bioremediation is being evaluated as a means for remediating a coal-tar contaminated aquifer at the site of a former manufactured gas plant. Two objectives of this work have been to evaluate the potential for the indigenous bacteria to biodegrade coal tar constituents and to identify factors controlling biodegradation rates. Aquifer sediments collected from a variety of locations across the site contain bacteria capable of aerobically mineralizing some of the principal aromatic compounds in the groundwater plume (benzene, naphthalene, and phenanthrene). Parallel mineralization assays incubated under aerobic and anaerobic conditions strongly suggest that O2 availability is a primary factor controlling the rate and extent of biodegradation. Data indicate that sorption may have also significantly affected biodegradation rates by limiting the bioavailability of the aromatic compounds. A mass transfer-limited numerical model was developed to explore the effect of sorption and bioavailability on biodegradation rates. In this model biodegradation rates are proportional to aqueous concentration, which is directly reduced by sorption. Both biotransformation and bacterial growth are described as being controlled by the rate of desorptive mass transfer. The influence of sorption on biodegradation is quantified by defining a Bioavailability Factor, Bf. A Thiele Modulus which indicates the ratio of characteristic times for sorption and biodegradation is helpful for determining the extent of mass transfer control during biodegradation of the aromatic compounds. This approach is preferred to equilibrium partitioning models, which may overestimate biodegradation rates by failing to consider the effect of rate-limited desorption on bioavailability.