Abstract: The solid-solid reaction of decomposing FeSO₄ from pyrites was studied.The effects of SO₂ and O₂ on the decompo- sition of FeSO₄ were investigated by HSC thermodynamic software simulation and experimental tests.Thermodynamic analysis indicated that SO₂ had no significant effect on the decomposition of FeSO₄，while O₂ could increase the complete-decomposi- tion temperature of FeSO₄.Experimental test showed that SO₂ would reduce the raw materials′ desulfurization rate，but further increased the concentration of SO₂ would not further affect the desulfurization rate of raw materials，this difference might re- sult from the fact that the HSC software has not considered the reaction equilibrium.The experiments also showed that the higher of oxygen concentration，the lower of raw material desulfurization rate，because the oxygen consumed part of pyrite which acted as the reductant in this reaction.

Key words: pyrite, ferrous sulfate, thermodynamic analysis, mixed atmosphere