a colourless pungent toxic gas formed by burning sulphur in air. Sulfur dioxide or sulphur dioxide is the chemical compound with the formula SO,₂. Sulfur dioxide or sulphur dioxide (British English) is the chemical compound with the formula SO 2. Sulfur dioxide, inorganic compound, a heavy, colorless, poisonous gas. Sulfur dioxide is a respiratory irritant that is highly soluble in the aqueous surfaces of the respiratory airways. Sulfur dioxide, or SO2 is a colorless gas with a strong odor, similar to a just-struck match.