Compounds in this group tend to be nonflammable.

In aqueous solution, thiosulfate hydrolyzes to sulfite. Sulfite and thiosulfate salts and their solutions are mild reducing agents. They liberate SOx gases when acidified. Sulfites will react with strong oxidizers or strong reducing agents such as hydrides, or alkali metals/active metals. With chlorosilanes/halogenating agents they release HX. They are known to react with epoxides in ring opening reactions. Reactions with isocyanates and sulfite addition reactions with aldehydes are known.

Sulfites are used in increasing amounts as food preservatives or enhancers. They may come in various forms. Sulfur dioxide, which is not a sulfite but can be a decomposition by-product, is a closely related chemical oxide. Sulfites are counted among the top nine food allergens. Some people (but not many) have positive skin allergy tests to sulfites indicating true (IgE-mediated) allergy. It may cause breathing difficulty within minutes after eating a food containing it; asthmatics and possibly people with salicylate sensitivity (or aspirin sensitivity) are at an elevated risk for reaction to sulfites. Anaphylaxis and life threatening reactions are rare.

The compounds covered in this group contain the sulfite ion, [SO3]2- , or thiosulfate ion, [S2O3]2-. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used. Sulfites occur naturally in all wines to some extent, with a few exceptions. Sulfites are commonly introduced to arrest fermentation at a desired time, and may also be added to wine as preservatives to prevent spoilage and oxidation at several stages of the winemaking. Sulfur dioxide (SO2, sulfur with two atoms of oxygen) protects wine from not only oxidation, but also bacteria. Without sulfites, grape juice would turn to vinegar. Thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. These materials commonly are used to initiate polymerization reactions by combining with an oxidizing agent (redox couples).

Potassium bisulfite, potassium metabisulfite, sodium bisulfite, sodium metabisulfite, sodium sulfite, lead thiosulfate, sodium thiosulfite, ammonium bisulfite, magnesium bisulfite.