Microorganisms exhibiting biological activity towards sulfur were screened for use in producing surface-modified ground tyre rubber (GTR) that exhibits improved compounding characteristics with virgin rubber stocks. The degradation of dibenzothiophene (DBT) was used as a model compound to screen strains ofThiobacillus,Rhodococcus andSulfolobus. The strains that were capable of degrading the model compounds were studied in more detail in small bench-scale degradation experiments, where the source of organically bound sulfur was finely ground waste tyre rubber (74 micron particle diameter). Ion chromatography (IC), Fourier transform infrared spectroscopy (FT-IR) and X-ray analysis of near-edge surfaces (XANES) were used to chemically characterize process effluents and products of these bioprocesses. IC was used to characterize spent processing media and showed that bound sulfur was released as sulfate ions when the bioreaction was allowed to run to completion, which agrees with the proposed progressive sulfur oxidation degradation model. The surface chemistry of biotreated GTR was characterized by infrared spectroscopy FT-IR and XANES. These analyses revealed that sulfur present in the GTR was oxidized, which corroborates the proposed biodegradative pathway. Physical properties were determined on rubber samples compounded with various loadings of the surface-modified GTR. Results showed significant increases in the tensile strength and elongation properties for compounds containing up to 15% GTR processed withS. acidocaldarius. The results of the screening experiments, bench-scale process optimization studies and physical properties characterization identifiedSulfolobus acidocaldarius as the best microorganism to use in developing a commercially viable bioprocess.