The presence of a fatty acid and an n-alkane may affect the biodegradation rate of aromatic sulphur compounds such as dibenzothiophene (DBT). A fatty acid (hexadecanoic acid) may form micellar structures favouring DBT bioavailability. n-Alkanes, such as n-dodecane or n-hexadecane, form a film around the aromatic sulphur molecule as a consequence of solvation, thus increasing DBT bioavailability. The mass-transfer rate from the solid to the aqueous phase controls the DBT biodegradation rate when DBT is the only carbon source. Diffusional coassimilation and microbial hydrophobic effects are rate-limiting steps in DBT biodegradation in the presence of aliphatic compounds. Diffusion depends on the DBT concentration in n-alkane, while cometabolism is associated with different n-alkane biodegradation rates. Through the definition of biodesulphurization selectivity and biodesulphurization efficiency, our investigations have shown that a selective aerobic biodesulphurization process is possible by using an unselective biocatalyst, such as a Pseudomonas sp.