Biotreatment of fossil fuels may be regarded as suitable technology for the desulfurization of oil and coal before combustion, especially for small-scale combustion plants where Rue gas cleaning is too expensive. Although several patents, especially on the field of biodesulfurization of oil, already exist, successful application in a large scale could not be demonstrated until now. ＇＇Organic suIfur＇＇ is characterized by covalent C-S bonds and can be regarded as an element which is integrated in the macromolecular matrix of coal and in organic sulfur compounds present in oil. As for the results of more semi-empirical approaches no reproducibility and transferability to other oil qualities could be demonstrated when model compounds representing the organic sulfur species were used. This research strategy resulted in a number of promising biochemical pathways for the degradation and/or desulfurization of such compounds. However, up to now the use of microorganisms able to degrade model compounds in a biotechnological process for coal or oil desulfurization does not meet the expectations. Thus, it has to be concluded that additional research is needed which should be focussed on the development of biocatalysts with a broad substrate specificity and on methods to improve the availability of the organically bound sulfur in the molecular structure of oil and coal.