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Oxyanion with a central atom of sulfur surrounded by 3 oxygen atoms

Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), SO32 The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used. Sulfites are substances that naturally occur in some foods and the human body. They are also used as regulated food additives. When in food or drink, sulfites are often lumped together with sulfur dioxide.

Sulfites occur naturally in all wines to some extent. 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) protects wine not only from oxidation, but also from bacteria. Organic wines are not necessarily sulfite free, but generally have lower amounts and regulations stipulate lower maximum sulfite contents for these wines. In general, white wines contain more sulfites than red wines and sweeter wines contain more sulfites than drier ones. In the United States, wines bottled after mid 1987 must have a label stating that they contain sulfites if they contain more than 10 parts per million (ppm). In the European Union an equivalent regulation came into force in November 2005. This includes sulfur dioxide, and the limit is on the milligrams per kilogram or per litre of sulfur dioxide equivalent. In the United Kingdom, similar laws apply. Bottles of wine that contain over 10 mg/L (ppm) of "sulfites" (or sulfur dioxide) are required to bear "contains sulphites" on the label. This does not differ if sulfites are naturally occurring or added in the winemaking process.

Sulfites are often used as preservatives in dried fruits, preserved radish, and dried potato products. Most beers no longer contain sulfites, although some alcoholic ciders contain them. Although shrimp are sometimes treated with sulfites on fishing vessels, the chemical may not appear on the label. In 1986, the Food and Drug Administration in the United States banned the addition of sulfites to all fresh fruit and vegetables that are eaten raw.

Allergic reactions to sulfites appear to be very rare in the general population, but more common in hyperallergic individuals. Sulfites are counted among the top nine food allergens, Some people have positive skin allergy tests to sulfites indicating true (IgE mediated) allergy. Chronic skin conditions in the hands, perineum, and face have been reported in individuals that regularly use cosmetics or medications containing sulfites. Occupational exposure to sulfites has been reported to cause persistent skin symptoms. 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. A 2017 study has shown negative impacts of sulfites on bacteria found in the human microbiome.

High sulfite content in the blood and urine of babies can be caused by molybdenum cofactor deficiency disease which leads to neurological damage and early death unless treated. Treatment, requiring daily injections, became available in 2009.