When CaS04 was treated with a mixture of CO, COz, SOz, and Nz at 1150 "C in thermogravimetric analysis equipment, various reaction stages were identified, and the final product distribution was found to depend on the reducing potential (Pco/PcoJ of the gases. At low reducing potentials (_<0.10), the CaS04 was completely converted to CaO. At moderate reducing potentials (Le., 0.20), CaS04 was converted rapidly to CaO, which was then converted more slowly to Cas. The rate-controlling mechanisms seemed to differ markedly for the earlier and final reaction stages.