Selective aerobic biodesulphurization is possible using an unselective biocatalyst, such as a Pseudomonas sp. The results were obtained in a model solution dissolving an aromatic sulphur compound, such as dibenzothiophene (DBT), in a poorly biodegradable solvent, such as dioctyl phthalate (DOG) or 2,2-4,4-6-8,8 heptamethylnonane (HMN). This system forces the microorganisms to work in ’obliged desulphurizing selectivity’ (ODS) conditions, as indicated by the values of biodesulphurization process selectivity which were 30-40% of effective organic phase biodesulphurization. ODS is a useful system for studying an ideal microbial desulphurization process and its limiting factors with the aim of developing a desulphurizing microorganism with high specificity The ODS process may also permit further work on the effects of different groups of compounds, such as n-alkanes and fatty acids which make up heavy oil, on the biodegradation rate of the aromatic sulphur compounds. The findings of this work show that diffusion, co-assimilation and microbial hydrophobic effects may account for the inhibition effects of n-alkanes and fatty acids on the biodegradation rare of DBT dissolved in DOC.