Growth assays reveal that Rhodococcus rhodochrous IGTSS uses a wide range of organosulphur compounds as the sole source of sulphur, yet none of the compounds serve as carbon sources. Compounds that are utilized include thiophenes, sulphides, disulphides, mercaptans, sulphoxides, and sulphones. A convenient spectrophotometric assay (Gibbs assay), based on the chromogenic reaction of 2,6-dichloroquinone-4-chloroimide with aromatic hydroxyl groups, was developed and used in conjunction with GC/MS analyses to examine the kinetics of dibenzothiophene metabolism by axenic and mixed cell cultures of Rhodococcus rhodochrous IGTSS. The desulphurization trait is expressed at increasing levels during the exponential phase of growth and then declines in stationary-phase cells. Mixtures of streptomycin-resistant Rhodococcus rhodochrous IGTSS and Enterobacter cloacae (an organism incapable of cleaving carbon-sulphur bonds in relevant test compounds) were prepared in ratios that varied over six orders of magnitude. Growth studies revealed that E. cloacae was able to gain access to sulphur liberated from organosulphur compounds by IGTSS ; however, cell-to-cell contact appears to be required. These experiments also indicate that the desulphurization activity, on a per cell basis, is higher in mixed cultures than in axenic cultures.