In the Molten Carbonate Process, the flue gases are scrubbed with a melt of alkali metal carbonates. This produces a melt mixture of alkali metal carbonates, sulfites, and sulfates, which must be treated to regenerate the carbonate and recover the sulfur. The first step of the treatment is the chemical reduction of the sulfite and sulfate to sulfide, using hydrogen, carbon monoxide, or carbon as the reducing agent. The chemistry of the hydrogen reduction step was studied in detail. It was found that the reduction was zero-order with respect to sulfate concentration, and that the rate was catalyzed by iron, and autocatalyzed by the sulfide product. Studies with carbon monoxide indicate it is not as reactive as hydrogen. However, its reactivity increases with pressure more rapidly than hydrogen, so it may be useful at high pressures. (Author)