A study of the feasibility of utilizing the Thiobacillus ferrooxidans or Desulfovibrio desulfuricans bacterium for microbial removal of sulfur dioxide from flue gases has been carried out. Sulfur dioxide may be readily reduced to H2S by contact with sulfate-reducing microorganisms in which Desulfovibrio desulfuricans were dominant in the first stage. The H2S was then oxidized to sulfur by the ferric sulfate in a second stage where ferrous ions were regenerated. These results were compared to microbial oxidation of SO2 from flue gases to sulfate by Thiobacillus ferrooxidans. The mechanisms for the reduction of SO2 to H2S in the presence of Desulfovibrio desulfuricans and the oxidation of SO2 to H2SO4 in the presence of ferric sulfate and Thiobacillus ferrooxidans are discussed. Sulfuric acid or gypsum CaSO4.2H(2)O are byproducts from microbial flue gas desulfurization.