Microbial degradation of hydrocarbons depends on various chemical and physical factors (viz, temperature, electron accepters, nutrients, pH, substrate characteristics) and on the presence of degrading microorganisms. Even if all these factors are optimized, oil biodegradation can still be retarded by diminished availability of the compound. Setti et al. (1992) have shown that n-alkanes affect the biodegradation of aromatic sulfur compounds by an aerobic Pseudomonas sp. This communication illustrates further investigations into the diffusion effect that controls the biodegradation of aromatic sulfur compounds. Polycyclic aromatic hydrocarbons (PAHs) can be converted in the dissolved state only. This implies that mass-transfer from the solid phase to the aqueous phase might be rate-limiting. Dibenzothiophene (DBT) biodegradation is a good example of this model. The presence of a fatty acid (hexadecanoic acid, aC16) during the fermentation affects DBT degradation. We propose a model in which aC16 surrounds the DBT molecule forming a micelle which favors a co-metabolic process between the fatty acid and the aromatic sulfur compound. Similar behavior is seen when DBT is dissolved in n-dodecane(C12) and n-hexadecane (C16). There are two important parameters that affect DBT degradation in this system: the surface area of the substrate and the DBT concentration in C12. DBT degradation is limited by the diffusion of the sulfur compound from the organic phase to the adsorbed microorganism on the hydrocarbon. Diffusion increases with the DBT concentration, and the DBT degradation rate is significantly higher than that reported for DBT alone. In this case, a carrier effect of the n-alkane in the DBT degradation is shown. A barrier effect is suggested when the DBT concentration is below the limit for which the degradation rate is lowest. Our investigations show that the presence of a co-substrate, such as fatty acids or n-alkanes, affects the bioavailability of the aromatic sulfur compound in aerobic conditions.