Abstract The study has been performed on samples of high sulphur content Spanish brown coal Mequinenza and Polish flame coal Czeczot. The samples of raw coals and the coals oxidised by four different oxidising agents have been subjected to derivative thermogravimetry analysis (DTG). The results have shown that demineralisation has no effect on the temperature range of the maximum mass loss. The Czeczot flame coal has been shown to be characterised by a narrower temperature range of the main devolatilisation peak (380–500 °C) than that of Mequinenza (320–500 °C). The most pronounced structural changes in coal have been found to occur as a result of its oxidation by 5% nitric acid. After such a process of oxidation a new peak of devolatilisation appears with a maximum at 265 °C, which can be assigned to the compounds having nitric groups in their structure.