A biodesulfurization process was carried out for two coals from the southwest of Colombia using a consortium of native microorganisms Acidithiobacillus ferrooxidans-like and Acidithiobacillus thiooxidans-like. Characterization techniques as X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (1717111), Scanning Electron Microscopy (SEM/EDX) and chemical analysis were used in order to establish the mineralogy of the process. The sulfur content in coals varies from 2.6% to 5.7%, and pyrite is the main sulphide present in the inorganic phase. Other minerals in the substrates include kaolinite, calcite, dolomite and quartz. An 85-95% reduction of pyritic sulfur and 31-51% of total sulfur in a period of 30 days were achieved, for a particle size of -74 mu m, 10% w/v pulp density and 30 degrees C. (C) 2009 Elsevier B.V. All rights reserved.