The Mössbauer effect has been used to study the transformations of FeS 2 in four different coals: IL No. 6, Ky 9 14 , Blacksville No. 2, and Powhatan No. 5. The transformations of FeS 2 in the coals were studied in an inert atmosphere. It was observed that the pyrrhotites formed from FeS 2 have a considerable reduction in the isomer shift at 440 °C as compared to the values obtained in the absence of coal. This effect is associated with the interaction of the pyrrhotites with the coal constituents at high temperatures. There is also a significant line-broadening at 440 °C. This broadening is due either to vacancy motion in the iron sulphides and/or to motional broadening due to particle motion in the coal-derived liquids. The percentage conversion of pyrite to pyrrhotite depends markedly on time as well as type of coal. The weathering of the coal has a detrimental effect on the rate of conversion of pyrite to pyrrhotite. The ferrous sulphate layers covering the pyrite particles hinder the removal of sulphur from that surface. The major factor affecting the Fe S ratio is the total amount of sulphur available for H 2S formation. Partial H 2S pressure is the crucial quantity controlling the stoichiometry of the pyrrhotites. Hence, a high percentage of H 2S in the reactor at high temperature will assure the formation of pyrrhotites with a high number of metal vacancies.