The kinetics for the reductive decomposition of calcium sulfate in the presence of carbon monoxide were investigated using thermogravimetric equipment under various conditions of temperature (1050-1200°C) and gas composition (0-7% carbon monoxide, 0-10% sulfur dioxide, and 10-50% carbon dioxide). Pellets reacted at selected conditions were withdrawn from the reaction system at various stages of the reaction and analyzed by X-ray powder diffraction, scanning electron microscopy with electron microprobe, and BET surface area analysis;In contrast to previous views of the reaction, the reductive decomposition of calcium sulfate was found to involve two separate reactions: (1) the reduction of calcium sulfate to calcium oxide and (2) the sulfidation of calcium oxide to calcium sulfide. When the reducing potential, P[subscript]co/P[subscript]co[subscript]2, was lower than 0.25, sulfidation did not appear to occur until the sulfate was almost completely converted to the oxide. The reduction of sulfate was found to take place simultaneously throughout a pellet, indicating negligible resistance to intra-pellet diffusion. On the other hand, sulfidation of the oxide seemed to follow a shrinking unreacted-core model;The kinetics for calcium sulfate reduction were notable for an initial induction period. The extremely slow rate of reaction during this period appeared to be controlled by the rate of nucleation of the calcium oxide reaction product;A mathematical model based on the Erofeev equation was developed to represent both the nucleation kinetics and the intrinsic gas-solid reaction kinetics. The rate of reduction was found to be first order with respect to carbon monoxide concentration and to have an activation energy of 479 kJ/mole. These parameters were compared with the results obtained by the application of the well-known grain model when the induction period was neglected;For the sulfidation of the oxide, a shrinking unreacted-core model of chemical reaction control was used to analyze the experimental data. The reaction was found to be first order with respect to carbon monoxide concentration, with an activation energy of 174 kJ/mole.