Biological denitrification typically requires the addition of a supplemental electron donor, which can add a significant operating expense to wastewater treatment facilities. Most common electron donors are organic, but reduced inorganic sulfur compounds (RISCs), such as sulfide (HS-) and elemental sulfur (S-0), may be more cost-effective. S-0 is an inexpensive and well characterized electron donor, but it provides slow denitrification rates due to its low solubility. A lesser-known RISC is sulfite (), which can be easily produced from S-0 by a simple combustion process. Unlike S-0, is highly soluble, and therefore may provide higher denitrification rates. However, very little is known about microbial denitrification with . Also, is a strong reductant that reacts abiotically with oxygen and has toxic effects on microorganisms. This paper reviews in the environment, chemistry, microbiology, toxicity, and its potential use for denitrification. Since is an intermediate in the sulfur oxidation pathway of most sulfur-oxidizing microorganisms, it is an energetic electron donor and it should select for a -oxidizing community. Our review of the literature, as well as our own lab experience, suggests that can effectively serve as an electron donor for denitrification. Further research is needed to determine the kinetics of -based denitrification, its toxic threshold for sulfur-oxidizing microorganisms, and its potential inhibition of sensitive species such as nitrifying microorganisms and potential formation of nitrous oxide. Its effect on sludge settling efficiency also should be explored.