Physical and biological processes have been combined with a view to reduce the sulphur and ash content of finely ground coal. The coal used was a semianthracite from the North Spain Coal Field. A sample of several kg in a coal/water suspension with a w/w concentration of 15% was subjected to a physical treatment combining cyclone and flotation separation processes. Representative samples were taken from each of five size fractions: original feed and the physical separation products, which were screened through water into sub-samples classified by particle size. Each fraction from the physical separation and the screening was analysed for ash and sulphur. The elimination yield of the hydrocyclone was 22% of ash and 21% of sulphur. The figure for ash raised to 41% when the hydrocyclone and flotation equipment were both used, with no change in sulphur elimination. In turn, the biodesulphurization treatment applied in addition to the two processes raised the respective yields to 59 and 42%. The change in the calorific value of the coal was hardly significant, while sulphur emissions, expressed as g S/GJ were reduced by 51%. Additionally, an assessment was made of the thermal behaviour of the various samples from the physical and biological treatment by means of programmed temperature combustion analysis carried out by a thermogravimetric equipment. (C) 2003 Elsevier Science Ltd. All rights reserved.