The purpose of this study was to improve the desulphurization yield of a high sulphur-content semianthracite from northern Spain adapted to the process conditions of a packed-bed reactor, for which three 5-kg packed columns of coal were set up with the pH stabilized at 1.5. The process was begun by inoculating the first column with a culture obtained from micro-organisms native to the coal itself, mainly Thiobacillus ferrooxidans and Leptospirillum ferrooxidans, grown in a stirred reactor. The percolate of this column was then used to seed the second column, the percolate of which was then inoculated into the last one. The purpose was to obtain from the last column a better adapted and more active biomass which would give a better desulphurization yield for coal. Results show an increased yield from the first column to the last over the first 20 days of treatment, but the total desulphurization in the third column was less than expected, owing to a high precipitation of jarosite. (C) 2003 Elsevier Ltd. All rights reserved.