Iron sulfides in the form of ferrimagnetic pyrrhotite have been removed from solvent refined coal by the use of high gradient magnetic separation. The results of such separations have been correlated with those of studies of the thermochemical and magnetochemical properties of the undissolved solids present in the product of coal liquefaction. Complete inorganic desulfurization was achieved at the same temperatures at which the solids exhibit a maximum magnetization, at about 230 °C. The preferred form for the highest magnetization is a monoclinic structure with a composition of Fe0.875S. Where the sulfides do not occur in this form, it has been possible to convert them to the high magnetization state by exposure to a hydrogen sulfide atmosphere at an elevated temperature. To approximate the temperature cycle experienced by the liquid coal coming from the reactor toward the filter, samples were heated above the Curie point to destroy the magnetization and then cooled to about 260 °C for H2S treatment. Measurements on the solids subsequently extracted showed a striking increase in the magnetization compared to the untreated material.