The reaction of sulfite reductase [EC 1.8.99.1] from Desulfovibrio vulgaris was investigated using a purified enzyme preparation in a system coupled with methyl viologen and hydrogenase [EC 1.12.2.1], Trithionate, thiosulfate, and sulfide were detected even in the early phase of sulfite reduction and the amount of each compound did not decrease during the reaction or after hydrogen uptake ceased. The specific radioactivity of sulfide formed from 85S-labelled sulfite was scarcely reduced on adding cold trithionate and only-slightly on adding cold thiosulfate. These results, in addition to the fact that trithionate and thiosulfate are not reduced by the enzyme, indicate that these three compounds are produced by sulfite reductase. At high concentrations of sulfite and low concentrations of methyl viologen or hydrogenase, trithionate was the dominant product. Under the opposite conditions, the formation of relatively large amounts of sulfide or thiosulfate was observed. On the basis of these findings, a mechanism was proposed for the reaction, including labile intermediates, presumably sulfoxylate and elemental sulfur, which accept electrons from reduced methyl viologen to form sulfur and sulfide or react with sulfite to produce trithionate and thiosulfafe, respectively. Some enzymatic properties were examined. Km was 3.6×lO−3M for sulfite. The optimum pH was 5.5 to 6.0. The enzyme was partially inhibited by arsenite at concentrations of 10−2 to 10−4M. It could reduce hydroxylamine, nitrite, and trimethylamine N-oxide, but not nitrate, chlorate, cyanide, azide, adenosine N-oxide, or taurine. Disc electrophoresis and enzyme staining using oxidation of reduced methyl viologen with acceptors in polyacrylamide gel revealed that desulfoviridin had reducing activities not only for sulfite but also for hydroxylamine and nitrite.