Abstract Differential thermogravimetry (DTG) was used to obtain information about the effect of biological desulphurisation on the entire coal combustion process. Two coals of different rank, a semianthracite (HVL1) and a high volatile bituminous coal (HM1), were inoculated with autochthonous bacterial cultures isolated from natural mine sludge and from bacteria inherent in the coal. From the combustion profiles and the reactivity in air at 500°C of the chars produced after pyrolysis, several characteristic parameters were determined. The results indicated that the influence of the biological treatment on combustion performance and char reactivity is more significant in the higher rank coal. The overall process seems to cause a slight beneficial effect on the ignition properties, evaluated from the burning profiles, of the semianthracite treated samples.