This research presents the first demonstration of substantial microbial degradation end depletion of naphthalene from coal tar, a multicomponent, aromatic, dense nonaqueous phase liquid (NAPL). The rates and extents of microbial degradation of naphthalene from coal tar and from a two-component NAPL simpler in composition than coal tar were evaluated in gently mixed, NAPL-water batch systems. The rate of degradation of naphthalene, the principal constituent in coal tar, was found to be significantly influenced by the rate of external surface mass transfer from the coal tar. Results show that the rate of mass transfer may control the overall rate of biotransformation in mixed systems where coal tar is present as a globule (≃11 mm diameter). Mass transfer is relatively rapid and does not limit biodegradation in slurry systems when coal tar is distributed among a large number of small microporous silica particles (≃250 μm diameter). These results were obtained for conditions favorable for biodegradation and provide an indication of the maximum potential rates for microbial degradation of naphthalene from coal tar. The microbial degradation process is dependent on relationships between the NAPL composition and the equilibrium aqueous naphthalene concentration, the naphthalene mass transfer rate between the NAPL and the aqueous phases, and the intrinsic rates of microbial degradation of naphthalene. These relationships have been incorporated in a dissolution- degradation framework, and the rate-limiting phenomena for the biodegradation process was evaluated using this framework.