Bromothymol blue is a member of the class of 2,1-benzoxathioles that is 2,1-benzoxathiole 1,1-dioxide in which both of the hydrogens at position 3 have been substituted by 3-bromo-4-hydroxy-5-isopropyl-2-methylphenyl groups. It has a role as an acid-base indicator, a dye and a two-colour indicator. It is a 2,1-benzoxathiole, an arenesulfonate ester, an organobromine compound, a polyphenol and a sultone.