Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula CH2(OH)2.

The compound is closely related and convertible to the industrially significant derivatives paraformaldehyde ((CH2O)n), formaldehyde (H2C=O), and 1,3,5-trioxane ((CH2O)3).

Gaseous methanediols can be generated by electron irradiation and sublimation of a mixture of methanol and oxygen ices.

[5] Methanediol, rather than formaldehyde, is listed as one of the main ingredients of "Brazilian blowout", a hair-straightening formula marketed in the United States.

[6][7] Research funded by the Professional Keratin Smoothing Council (PKSC), an industry association that represents selected manufacturers of professional-use only keratin smoothing products, has disputed the risk.