Calcium sulfate is decomposed to CaO and SO₂ by high temperature treatment in a fluidized bed wherein reductive and oxidative conditions are simultaneously maintained. A highly reducing gas is formed in the lower portion of the bed from partial combustion of the fuel in admixture with the primary fluidizing air. The quantity of the primary fluidizing air is limited so that the reducing conditions in the lower zone converts the CaSO₄ to a mixture of CaO and CaS. Secondary air is introduced at a higher level in the bed to create an oxidizing zone in the upper portion of the bed above the reducing zone capable of converting CaS to CaO. The concurrent use of such reducing and oxidizing zones permits reducing conditions to be maintained which favor a high rate of decomposition even though these conditions favor the formation of CaS as well as CaO. The undesirable CaS, which would otherwise be discharged with the CaO product is eliminated by circulation of the fluidized particles through the oxidizing zone. Further, the heat of the exothermic reactions is conserved and utilized for promoting the endothermic reactions, both types of reactions occuring simultaneously while the rapid fluidized circulation of solids maintains a relatively uniform temperature throughout the bed.