The investigations into the dielectric heating characteristics of pyrite and marcasite found that microwave power, crystal structure and particle size had an effect on dielectric heating rate. Marcasite could be heated at a higher rate than pyrite. The studies exhibited that marcasite and pyrite could be decomposed by dielectric heating in an atmosphere of nitrogen. Marcasite was more readily decomposed than pyrite. Microscopic examinations and X-ray diffraction analysis showed that both the minerals were decomposed into pyrrhotite (Fe1-xS) and elemental sulphur.