A new process for converting SO2 to elemental sulfur by a cyclic process involving CaS and CaSO4 has been developed. In this process, the raw material CaSO4 is reduced to produce CaS, which is used to reduce SO2 to elemental sulfur and produce CaSO4. The latter is then reduced to regenerate CaS. Experimentally, about 75% of CaS powder was converted to CaSO4 in 20 min at 1153 K under 25.8 kPa of SO2 partial pressure. About 95% of nickel-catalyzed CaSO4 powder was reduced to CaS in 20 min at 1123 K under 86.1 kPa of hydrogen partial pressure. The reactivities of the powders remained relatively unchanged after 10 cycles of reactions and regenerations. Sulfur dioxide containing streams from certain sources contain higher partial pressures of SO2, in, which case the SO2-CaS reaction will be proportionally faster than that reported here. Detailed kinetics of the two-component reactions are described in the accompanying parts of this series.