This study examines the role of physico-chemical mass transfer processes on the rate of biotransformation of polycyclic aromatic hydrocarbon (PAH) compounds released from non-aqueous phase liquid (NAPL) coal tar present at residual saturation within a microporous medium. A simplified coupled dissolution-degradation model is developed that describes the concurrent mass transfer and biokinetic processes occurring in the system. Model results indicate that a dimensionless Damkohler number can be utilized to distinguish between systems that are mass transfer limited, and those that are limited by biological phenomena. The Damkohler number is estimated from independent laboratory experiments that measure the rates of aqueous phase dissolution and biodegradation of naphthalene from coal tar. Experimental data for Stroudsburg coal tar imbibed within 236 μm diameter silica particles yield Damkohler numbers smaller than unity, indicating, for the particular system under study, that the overall rate of biotransformation of naphthalene is not limited by the mass transfer of naphthalene from coal tar to the bulk aqueous phase. There is a need for investigation of mass transfer for larger particles and/or other PAH compounds, and study of microbial rate-limiting phenomena including toxicity, inhibition and competitive substrate utilization.