Thiodiglycol, or bis(2-hydroxyethyl)sulfide (also known as 2,2-thiodiethanol or TDE), is the organosulfur compound with the formula S(CH2CH2OH)2. It is miscible with water and polar organic solvents. It is a colorless liquid. It is structurally similar to diethylene glycol.
Thiodiglycol is used as a mounting medium in microscopy. The ability to vary the refractive index of the medium by varying the concentration of TDE in an aqueous solution, plus its relative lack of toxicity makes it highly desirable for such use. The refractive index of the solution can be varied anywhere from near that of water (1.333) to that of glass (1.518).[2]