This chapter focuses on low-spin sulfite reductases, which are isolated from Desulfovibrio vulgaris (Hildenborough) (DvH), Methanosarcina barkeri (Ms. Barkeri), and Desulforomonas acetoxidans (Drm. Acetoxidans). Sulfite reductase catalyzes the six-electron reduction of SO32- to S2-. This enzyme contains an iron tetrahydroporphyrin prosthetic group, termed “siroheme,” in addition to nonheme iron. On the basis of physiological function, two types of sulfite reductases can be defined (1) the assimilatory type, which is involved in the synthesis of sulfur-containing compounds, and (2) the dissimilatory one, which participates in the respiratory pathway for sulfate-reducing bacteria (SRB). Under certain assay conditions of dissimilatory sulfite reductases, in addition to sulfide, trithionate and thiosulfate are irreversibly produced. All purification procedures of sulfite reductases are carried out at 4〫 and all buffers are adjusted to pH 7.6. The purified enzyme is stable for six months in 0.05 M Tris-HCl buffer at –80〫. The purified sulfite reductase has a molecular mass of 27.2 kDa, determined by amino-acid composition. The iron content is consistent with the existence of a single [4Fe-4S] cluster and a siroheme.