Rhodococcus sp. strain IGTS8 (ATCC 53968) is capable of removing organic sulphur from various organosulphur compounds such as dibenzothiophene (DBT). Since substrates for this process are invariably hydrophobic, parameters of the biodesulphurisation of DBT in hydrophobic systems were examined. Freeze-dried bacteria, stored for 3 months at -80 degrees C with negligible loss of activity were used primarily as biocatalysts. The most efficacious order of ingredient addition was with oil and water (plus surfactant if used) mixed first and then freeze-dried bacteria added. Various oil/water ratios were examined. The minimum water requirement for desulphurisation was 1.25 cm(3) g(-1) dry weight. If the minimum water requirement was met, biodesulphurisation at an 90% oil/water (w/w) ratio was still 82% of the maximum at an 80% oil/water ratio. The surfactants, oleic diethanolamine and Triton N101, both stimulated biodesulphurisation in oil/water systems but not in 100% water. The desulphurisation of DBT in oil/water emulsions persisted for 8-16 h, but the rate rapidly declined after 3 h; biodesulphurisation in aqueous media stopped after 3-4 h.