The bioavailability of naphthalene present as a component of a complex nonaqueous phase liquid (NAPL) comprised by nine aromatic compounds was investigated. Specifically, the effects of naphthalene mass transfer from the NAPL to the aqueous phase on rates of its microbial degradation were examined. The investigations were conducted using a pure culture, ATCC 17484, and a mixed culture of naphthalene-degrading bacteria, the former having been implicated previously in the direct uptake of sorbed naphthalene. The studies were conducted in mass-transfer-limited, segregated-phase reactors (SPRs) in which both the NAPL and aqueous phases were internally well-mixed. A 30-day active biodegradation period was preceded and followed by a 5-7-day period devoid of bioactivity, during which time the rates and extents of mass transfer of components from the NAPL to the aqueous phase were quantified. The NAPL-phase naphthalene mass depletion profiles during biodegradation were compared to those predicted by assuming maximum mass depletion under mass-transfer-limited conditions using both pre- and post-biodegradation dissolution rate and equilibrium parameters. The observed mass depletion rates were high during the initial stages of biodegradation but decreased significantly in later stages. Throughout biodegradation, even in the initial rapid stage, mass depletion rates never exceeded maximum predicted rates based on pre-biodegradation mass transfer parameters. Reduced depletion rates in the later stages appear to relate to mass transfer hindrance caused by formation of biofilms at the NAPL-water interface.