β-Sulphopropionic acid has been isolated from the reaction of glucose or fructose with sodium sulphite. The source of this acid has been studied by using glucose-1-, -2-, and -6-[14C]. A sulphonic acid, probably 3,4-dideoxy-4-sulpho-D-hexosone, has been isolated from the reaction of glucose with sulphite at pH 6.5. The mechanism of the formation of this acid is discussed.