An experimental study of the oxidation of Indian natural pyrites is presented here. Three pyrite samples, two obtained directly as mineral fractions from the ores of Amjhore and Deri and third as a constituent of coal from Talchir, have been studied by Mössbauer spectroscopy after heating at different temperatures in air. Mineralogical, chemical and infrared absorption analyses have also been carried out. In all three cases, the first product of oxidation observed is sulphate instead of α-Fe 2O 3. The oxidation is seen to proceed in the sequence FeS 2 → FeSO 4 · 7 H 2O → α-Fe 2O 3 and/or Fe 2(SO 4) 3 → α-Fe 2O 3. The start of the thermal decomposition in these samples has been found to be governed by the degree of crystallinity, whereas complete conversion to α-Fe 2O 3 is observed to depend upon the particle size and the carbonaceous matter and sulfur contents.